feat(03-05): add write client calls and eventForm Zustand keys

- createEvent(payload): POST /api/events/create, credentials:include, returns {uid}
- updateEvent(uid, payload): PATCH /api/events/:uid/edit
- fetchWritableCalendars(): GET /api/events/writable-calendars, returns calendars array (D-03 server-authoritative)
- Exported interfaces: CreateEventPayload, CreateEventResponse, WritableCalendar, RecurrencePreset
- calendarStore: eventFormOpen (bool), eventFormMode ('create'|'edit'), eventFormUid (string|null)
- setEventForm(open, mode?, uid?) setter with correct defaults

apps/pwa/src/store/calendarStore.ts

@@ -35,10 +35,27 @@ export interface CalendarStore {
   openEventId: string | null
   calendarRange: { start: string; end: string }
 
+  // ── EventForm UI state (Plan 03-05) ──────────────────────────────────────
+  // Server state (events, calendars) lives in TanStack Query. These are pure
+  // UI-shape keys: whether the form is open, what mode it is in, and which
+  // event UID to pre-populate in edit mode.
+  eventFormOpen: boolean
+  eventFormMode: 'create' | 'edit'
+  eventFormUid: string | null
+
   setSelectedView: (view: string) => void
   setSelectedDate: (date: string) => void
   setOpenEventId: (id: string | null) => void
   setCalendarRange: (range: { start: string; end: string }) => void
+
+  /**
+   * Open or close the EventForm.
+   *
+   * @param open   true to open, false to close
+   * @param mode   'create' (default) or 'edit'
+   * @param uid    UID of the event to pre-populate in edit mode (null otherwise)
+   */
+  setEventForm: (open: boolean, mode?: 'create' | 'edit', uid?: string | null) => void
 }
 
 // ── Helpers ────────────────────────────────────────────────────────────────
@@ -93,6 +110,11 @@ export const useCalendarStore = create<CalendarStore>((set) => ({
   openEventId: null,
   calendarRange: initialCalendarRange(),
 
+  // EventForm UI state — closed by default, create mode, no pre-fill UID
+  eventFormOpen: false,
+  eventFormMode: 'create',
+  eventFormUid: null,
+
   setSelectedView: (view: string) => {
     set({ selectedView: view })
     // Persist to localStorage keyed by breakpoint group
@@ -110,4 +132,7 @@ export const useCalendarStore = create<CalendarStore>((set) => ({
 
   setCalendarRange: (range: { start: string; end: string }) =>
     set({ calendarRange: range }),
+
+  setEventForm: (open: boolean, mode: 'create' | 'edit' = 'create', uid: string | null = null) =>
+    set({ eventFormOpen: open, eventFormMode: mode, eventFormUid: uid }),
 }))