feat(04-04): add ListDetail with active/completed split + ItemRow + AddItemInput (LIST-02)

- listsClient.ts: add fetchListItems, addItem, patchListItem, deleteItem + ListItemsResponse type
- ListDetail.tsx: replace placeholder with real implementation — useQuery(['list', listId])
  with 30s polling fallback (D-12); active/completed split (D-05); optimistic mutations (D-07);
  delete-wins no-rollback (D-09); per-field check PATCH (D-08)
- ItemRow.tsx: 44px touch target, checkbox (20px visual/44px touch, accent fill when checked),
  plain-text item text (T-04-06 XSS guard), GripVertical handle slot for Plan 05,
  hover Trash2 delete + swipe-left zone, transform 150ms ease-out animation slot (D-14)
- AddItemInput.tsx: sticky bottom input + Add button, disabled when empty, Enter key support
- ListDetail.test.tsx: 7 real tests replacing todo stubs — optimistic add/check/uncheck/delete,
  rollback on error, D-05 completed-sink split, D-09 delete-wins no-rollback
- Playwright browser check: add milk → sinks to Completed on check → vanishes on delete PASS

apps/pwa/src/routes/ListDetail.tsx

@@ -1,14 +1,37 @@
 /**
  * ListDetail — Single list view (/lists/:listId).
  *
- * PLACEHOLDER: This is a stub route component for Plan 04-01.
- * The full implementation (items, SSE, drag-to-reorder, etc.) is built in Plan 04-04.
+ * Replaces the Plan 04-01 placeholder. Delivers LIST-02:
+ *   - Fetches items via useQuery(['list', listId]) with 30s polling fallback (D-12)
+ *   - Splits items into active (!checked, sorted rank ASC) and completed (checked) (D-05)
+ *   - Optimistic mutations for add / check / delete (D-07/D-09)
+ *   - Delete-wins: no rollback on item delete (D-09)
+ *   - Per-field LWW PATCH: check/uncheck sends { checked } only (D-08)
  *
- * Purpose: allows react-router's /lists/:listId route to resolve without error
- * so the BottomTabBar NavLink and any deep-link won't 404.
+ * Live sync (SSE) is wired in Plan 06.
+ * Drag-to-reorder (dnd-kit) is wired in Plan 05.
+ *
+ * Security: item text rendered as plain-text JSX children (T-04-06 XSS guard).
  */
 
-export function ListDetail() {
+import { useState } from 'react'
+import { useParams, useNavigate } from 'react-router'
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import { ChevronLeft } from 'lucide-react'
+import {
+  fetchListItems,
+  addItem,
+  patchListItem,
+  deleteItem,
+} from '../api/listsClient.js'
+import { ItemRow } from '../components/ItemRow.js'
+import { AddItemInput } from '../components/AddItemInput.js'
+import type { ListItem, ListItemsResponse } from '../api/listsClient.js'
+
+/**
+ * ListEmptyState — shown inside ListDetail when list has no items.
+ */
+function ListEmptyState() {
   return (
     <div
       style={{
@@ -16,13 +39,13 @@ export function ListDetail() {
         flexDirection: 'column',
         alignItems: 'center',
         justifyContent: 'center',
-        height: '100%',
-        gap: '12px',
-        fontFamily: 'var(--font-family-base)',
-        padding: '0 var(--space-4, 16px)',
+        flex: 1,
+        gap: 'var(--space-3)',
+        padding: 'var(--space-6) var(--space-4)',
         textAlign: 'center',
-        paddingBottom: 'calc(56px + env(safe-area-inset-bottom, 0px))',
+        fontFamily: 'var(--font-family-base)',
       }}
+      aria-live="polite"
     >
       <div
         style={{
@@ -31,16 +54,310 @@ export function ListDetail() {
           color: 'var(--color-text-primary)',
         }}
       >
-        List view coming soon
+        Nothing here yet
       </div>
       <div
         style={{
           fontSize: 'var(--text-body-size, 15px)',
           color: 'var(--color-text-muted)',
+          maxWidth: '240px',
         }}
       >
-        Full list detail with live sync is being built in Phase 4 Plan 4.
+        Add your first item below.
       </div>
     </div>
   )
 }
+
+export function ListDetail() {
+  const { listId } = useParams<{ listId: string }>()
+  const navigate = useNavigate()
+  const queryClient = useQueryClient()
+  const parsedListId = Number(listId)
+
+  const [completedExpanded, setCompletedExpanded] = useState(true)
+
+  // ── Data fetch ─────────────────────────────────────────────────────────────
+  // refetchInterval: 30000 = D-12 polling fallback (always active; SSE layered in Plan 06)
+  const { data, isLoading, isError } = useQuery({
+    queryKey: ['list', parsedListId],
+    queryFn: () => fetchListItems(parsedListId),
+    refetchInterval: 30_000,
+    enabled: !isNaN(parsedListId),
+  })
+
+  // Split into active (unchecked, sorted rank ASC) and completed (checked) per D-05
+  const allItems: ListItem[] = data?.items ?? []
+  const activeItems = allItems
+    .filter((i) => !i.checked)
+    .sort((a, b) => (a.rank < b.rank ? -1 : a.rank > b.rank ? 1 : 0))
+  const completedItems = allItems.filter((i) => i.checked)
+
+  // ── Add item mutation (D-07 optimistic: append at active bottom, opacity 0.6) ──
+  const addMutation = useMutation({
+    mutationFn: (text: string) => addItem(parsedListId, { text }),
+    onMutate: async (text: string) => {
+      await queryClient.cancelQueries({ queryKey: ['list', parsedListId] })
+      const previous = queryClient.getQueryData<ListItemsResponse>(['list', parsedListId])
+
+      // Compute optimistic rank: after the last active item
+      const lastRank = activeItems.at(-1)?.rank ?? null
+      const { generateKeyBetween } = await import('fractional-indexing')
+      const optimisticRank = generateKeyBetween(lastRank, null)
+
+      const optimisticItem: ListItem = {
+        id: -Date.now(), // temporary negative id
+        listId: parsedListId,
+        text,
+        checked: false,
+        rank: optimisticRank,
+      }
+
+      queryClient.setQueryData<ListItemsResponse>(['list', parsedListId], (old) => ({
+        items: [...(old?.items ?? []), optimisticItem],
+      }))
+
+      return { previous, optimisticItem }
+    },
+    onError: (_err, _text, context) => {
+      // Rollback on rejection
+      if (context?.previous) {
+        queryClient.setQueryData(['list', parsedListId], context.previous)
+      }
+    },
+    onSettled: () => {
+      queryClient.invalidateQueries({ queryKey: ['list', parsedListId] })
+    },
+  })
+
+  // ── Check/uncheck mutation (D-07 optimistic: move section immediately, rollback on error) ──
+  const checkMutation = useMutation({
+    mutationFn: ({ itemId, checked }: { itemId: number; checked: boolean }) =>
+      patchListItem(itemId, { checked }),
+    onMutate: async ({ itemId, checked }) => {
+      await queryClient.cancelQueries({ queryKey: ['list', parsedListId] })
+      const previous = queryClient.getQueryData<ListItemsResponse>(['list', parsedListId])
+
+      queryClient.setQueryData<ListItemsResponse>(['list', parsedListId], (old) => ({
+        items: (old?.items ?? []).map((item) =>
+          item.id === itemId ? { ...item, checked } : item,
+        ),
+      }))
+
+      return { previous }
+    },
+    onError: (_err, _vars, context) => {
+      if (context?.previous) {
+        queryClient.setQueryData(['list', parsedListId], context.previous)
+      }
+    },
+    onSettled: () => {
+      queryClient.invalidateQueries({ queryKey: ['list', parsedListId] })
+    },
+  })
+
+  // ── Delete item mutation (D-09 delete-wins: NO rollback on error) ──
+  const deleteMutation = useMutation({
+    mutationFn: (itemId: number) => deleteItem(itemId),
+    onMutate: async (itemId: number) => {
+      await queryClient.cancelQueries({ queryKey: ['list', parsedListId] })
+      // Remove item optimistically — no previous state saved (delete-wins D-09)
+      queryClient.setQueryData<ListItemsResponse>(['list', parsedListId], (old) => ({
+        items: (old?.items ?? []).filter((item) => item.id !== itemId),
+      }))
+    },
+    // No onError rollback — delete-wins (D-09)
+    onSettled: () => {
+      queryClient.invalidateQueries({ queryKey: ['list', parsedListId] })
+    },
+  })
+
+  // ── Handlers ───────────────────────────────────────────────────────────────
+
+  function handleAdd(text: string) {
+    addMutation.mutate(text)
+  }
+
+  function handleCheck(itemId: number, checked: boolean) {
+    checkMutation.mutate({ itemId, checked })
+  }
+
+  function handleDelete(itemId: number) {
+    deleteMutation.mutate(itemId)
+  }
+
+  // ── Loading / error states ─────────────────────────────────────────────────
+
+  if (isNaN(parsedListId)) {
+    return (
+      <div
+        style={{
+          padding: 'var(--space-4)',
+          fontFamily: 'var(--font-family-base)',
+          color: 'var(--color-text-primary)',
+        }}
+      >
+        Invalid list.
+      </div>
+    )
+  }
+
+  // ── Render ─────────────────────────────────────────────────────────────────
+
+  return (
+    <div
+      style={{
+        display: 'flex',
+        flexDirection: 'column',
+        height: '100%',
+        background: 'var(--color-surface)',
+        fontFamily: 'var(--font-family-base)',
+        paddingBottom: 'calc(56px + env(safe-area-inset-bottom, 0px))',
+      }}
+    >
+      {/* Header */}
+      <div
+        style={{
+          display: 'flex',
+          alignItems: 'center',
+          gap: 'var(--space-2)',
+          padding: 'var(--space-3) var(--space-4)',
+          borderBottom: '1px solid var(--color-border)',
+          minHeight: '56px',
+        }}
+      >
+        <button
+          onClick={() => navigate('/lists')}
+          aria-label="Back to lists"
+          style={{
+            background: 'none',
+            border: 'none',
+            cursor: 'pointer',
+            padding: 'var(--space-2)',
+            color: 'var(--color-text-primary)',
+            display: 'flex',
+            alignItems: 'center',
+            minWidth: '44px',
+            minHeight: '44px',
+          }}
+        >
+          <ChevronLeft size={20} aria-hidden="true" />
+        </button>
+        <h1
+          style={{
+            flex: 1,
+            fontSize: 'var(--text-heading-size, 18px)',
+            fontWeight: 600,
+            color: 'var(--color-text-primary)',
+            margin: 0,
+          }}
+        >
+          {/* List name not available without an extra fetch; shown as placeholder.
+              Plan 05/06 can enrich from the ['lists'] cache by listId. */}
+          List
+        </h1>
+      </div>
+
+      {/* Content area */}
+      <div
+        style={{
+          flex: 1,
+          overflowY: 'auto',
+          display: 'flex',
+          flexDirection: 'column',
+        }}
+      >
+        {isLoading && (
+          <div
+            style={{
+              padding: 'var(--space-6) var(--space-4)',
+              color: 'var(--color-text-muted)',
+              textAlign: 'center',
+              fontSize: 'var(--text-body-size, 15px)',
+            }}
+          >
+            Loading…
+          </div>
+        )}
+
+        {isError && (
+          <div
+            style={{
+              padding: 'var(--space-6) var(--space-4)',
+              color: 'var(--color-destructive)',
+              textAlign: 'center',
+              fontSize: 'var(--text-body-size, 15px)',
+            }}
+          >
+            Failed to load items.
+          </div>
+        )}
+
+        {!isLoading && !isError && allItems.length === 0 && <ListEmptyState />}
+
+        {/* Active items section */}
+        {activeItems.length > 0 && (
+          <div role="list" aria-label="Active items">
+            {activeItems.map((item) => (
+              <div key={item.id} role="listitem">
+                <ItemRow
+                  item={item}
+                  isActive={true}
+                  onCheck={handleCheck}
+                  onDelete={handleDelete}
+                  // Newly optimistic items have negative id and dim opacity
+                  optimisticOpacity={item.id < 0 ? 0.6 : 1}
+                />
+              </div>
+            ))}
+          </div>
+        )}
+
+        {/* Completed section (D-05) — collapsible, default expanded */}
+        {completedItems.length > 0 && (
+          <div>
+            <button
+              onClick={() => setCompletedExpanded((prev) => !prev)}
+              style={{
+                width: '100%',
+                background: 'none',
+                border: 'none',
+                borderTop: activeItems.length > 0 ? '1px solid var(--color-border)' : 'none',
+                padding: 'var(--space-3) var(--space-4)',
+                display: 'flex',
+                alignItems: 'center',
+                cursor: 'pointer',
+                color: 'var(--color-text-muted)',
+                fontSize: 'var(--text-label-size, 13px)',
+                fontWeight: 400,
+                textAlign: 'left',
+                minHeight: '44px',
+              }}
+              aria-expanded={completedExpanded}
+            >
+              Completed ({completedItems.length})
+            </button>
+
+            {completedExpanded && (
+              <div role="list" aria-label="Completed items">
+                {completedItems.map((item) => (
+                  <div key={item.id} role="listitem">
+                    <ItemRow
+                      item={item}
+                      isActive={false}
+                      onCheck={handleCheck}
+                      onDelete={handleDelete}
+                    />
+                  </div>
+                ))}
+              </div>
+            )}
+          </div>
+        )}
+      </div>
+
+      {/* Sticky add-item input */}
+      <AddItemInput onAdd={handleAdd} isPending={addMutation.isPending} />
+    </div>
+  )
+}