fix(18): enable first-run timezone save when not explicitly set (WR-01)

- Derive isExplicit from timezoneQuery.data?.isExplicitlySet
- Apply the input===stored no-op guard only when isExplicit is true
- Keep pending and empty-input guards unconditional
- Add unit tests (AdminPage.timezone.test.ts) verifying first-run Save is
  enabled when isExplicitlySet:false and input matches stored fallback value

apps/pwa/src/routes/AdminPage.timezone.test.ts

@@ -0,0 +1,124 @@
+/**
+ * AdminPage timezone Save-button disabled logic — unit tests (WR-01 fix verification)
+ *
+ * Tests the `timezoneSaveDisabled` derivation inline:
+ *
+ *   const timezoneSaveDisabled =
+ *     timezoneMutation.isPending ||
+ *     effectiveTimezoneInput === '' ||
+ *     (isExplicit && effectiveTimezoneInput === storedTimezone);
+ *
+ * The pre-fix bug: Save was always disabled when input === stored value,
+ * regardless of isExplicitlySet. On first run (isExplicitlySet: false) this meant
+ * the admin could never confirm/save the displayed system-default zone.
+ */
+
+import { describe, it, expect } from 'vitest';
+
+/**
+ * Pure function extracted from AdminPage that computes whether Save should be disabled.
+ * This mirrors the logic that must be present post-fix in AdminPage.tsx.
+ */
+function computeTimezoneSaveDisabled({
+  isPending,
+  effectiveTimezoneInput,
+  storedTimezone,
+  isExplicit,
+}: {
+  isPending: boolean;
+  effectiveTimezoneInput: string;
+  storedTimezone: string;
+  isExplicit: boolean;
+}): boolean {
+  return (
+    isPending ||
+    effectiveTimezoneInput === '' ||
+    (isExplicit && effectiveTimezoneInput === storedTimezone)
+  );
+}
+
+// ---------------------------------------------------------------------------
+// Pre-fix behavior: the bug that WR-01 identified
+// ---------------------------------------------------------------------------
+
+describe('timezoneSaveDisabled — WR-01 first-run (isExplicitlySet: false)', () => {
+  it('Save is ENABLED when not explicitly set, even if input matches stored value (WR-01)', () => {
+    // First run: GET returns { timezone: 'UTC', isExplicitlySet: false }
+    // Input pre-fills to 'UTC'. This must enable Save so admin can confirm.
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: false,
+      effectiveTimezoneInput: 'UTC',
+      storedTimezone: 'UTC',
+      isExplicit: false,
+    });
+    expect(disabled).toBe(false); // WR-01: was `true` before fix (bug)
+  });
+
+  it('Save is ENABLED when not explicitly set with a real zone matching stored', () => {
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: false,
+      effectiveTimezoneInput: 'America/Chicago',
+      storedTimezone: 'America/Chicago',
+      isExplicit: false,
+    });
+    expect(disabled).toBe(false);
+  });
+
+  it('Save is DISABLED when pending (regardless of isExplicit)', () => {
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: true,
+      effectiveTimezoneInput: 'UTC',
+      storedTimezone: 'UTC',
+      isExplicit: false,
+    });
+    expect(disabled).toBe(true);
+  });
+
+  it('Save is DISABLED when input is empty', () => {
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: false,
+      effectiveTimezoneInput: '',
+      storedTimezone: 'UTC',
+      isExplicit: false,
+    });
+    expect(disabled).toBe(true);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Already-explicit cases: no-op re-saves should stay disabled
+// ---------------------------------------------------------------------------
+
+describe('timezoneSaveDisabled — already explicitly set (isExplicitlySet: true)', () => {
+  it('Save is DISABLED when input matches stored and timezone is explicit (no-op guard)', () => {
+    // Admin previously saved 'America/Chicago'. Input still shows 'America/Chicago'.
+    // No change → Save stays disabled (two-tap UX).
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: false,
+      effectiveTimezoneInput: 'America/Chicago',
+      storedTimezone: 'America/Chicago',
+      isExplicit: true,
+    });
+    expect(disabled).toBe(true);
+  });
+
+  it('Save is ENABLED when input differs from stored (user changed value)', () => {
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: false,
+      effectiveTimezoneInput: 'Europe/Paris',
+      storedTimezone: 'America/Chicago',
+      isExplicit: true,
+    });
+    expect(disabled).toBe(false);
+  });
+
+  it('Save is DISABLED when pending even if value differs', () => {
+    const disabled = computeTimezoneSaveDisabled({
+      isPending: true,
+      effectiveTimezoneInput: 'Europe/Paris',
+      storedTimezone: 'America/Chicago',
+      isExplicit: true,
+    });
+    expect(disabled).toBe(true);
+  });
+});

apps/pwa/src/routes/AdminPage.tsx

@@ -111,11 +111,21 @@ export function AdminPage() {
   const storedTimezone = timezoneQuery.data?.timezone ?? '';
   const effectiveTimezoneInput = timezoneInput ?? storedTimezone;
 
-  // Save is disabled when pending, or when input matches what's stored
+  // WR-01: derive isExplicit so we only apply the no-op guard when the timezone
+  // has ALREADY been explicitly saved. On first run (isExplicitlySet: false) the
+  // admin must be able to confirm/save the displayed system-default — even if the
+  // input value already matches the fallback string. Keep pending and empty-input
+  // guards unconditional.
+  const isExplicit = timezoneQuery.data?.isExplicitlySet ?? false;
+
+  // Save is disabled when:
+  //   - mutation is in-flight (pending), OR
+  //   - input is empty, OR
+  //   - the timezone IS already explicitly set AND the input is unchanged (no-op)
   const timezoneSaveDisabled =
     timezoneMutation.isPending ||
     effectiveTimezoneInput === '' ||
-    effectiveTimezoneInput === storedTimezone;
+    (isExplicit && effectiveTimezoneInput === storedTimezone);
 
   // IANA zones list for the datalist (Intl.supportedValuesOf may not be present in all runtimes)
   const ianaZones: string[] =