feat(03-06): implement SyncStateToast with polled sync-status feedback (D-05/D-06/D-08/D-09)

- SyncStateToast: pending/done/failed/dead states per UI-SPEC
- refetchInterval 3000ms while pending; disabled on terminal status
- done + conflict (412) invalidate ['events'] cache (D-06/D-08)
- done auto-dismisses after 2s; failed/dead persist with dismiss button
- role=status (pending/done) and role=alert (failed/dead) for a11y
- Mounted in CalendarShell (both phone + tablet/desktop layouts)
- EventForm.onSuccess: setLastSyncedUid(uid) instead of invalidateQueries

apps/pwa/src/components/EventForm.tsx

@@ -104,6 +104,7 @@ function parseDateTime(iso: string): { date: string; time: string } {
 
 export function EventForm() {
   const { eventFormOpen, eventFormMode, eventFormUid, setEventForm } = useCalendarStore()
+  const setLastSyncedUid = useCalendarStore((s) => s.setLastSyncedUid)
   const queryClient = useQueryClient()
   const titleRef = useRef<HTMLInputElement>(null)
 
@@ -190,8 +191,10 @@ export function EventForm() {
       eventFormMode === 'edit' && eventFormUid
         ? updateEvent(eventFormUid, payload)
         : createEvent(payload),
-    onSuccess: () => {
-      queryClient.invalidateQueries({ queryKey: ['events'] })
+    onSuccess: (data) => {
+      // Wire sync-toast: set the returned UID so SyncStateToast starts polling (D-05/D-09)
+      setLastSyncedUid(data.uid)
+      // Do NOT invalidate here — SyncStateToast does it on done/conflict (D-06/D-08)
       if (calendarUrl) writeLastCalendarUrl(calendarUrl)
       setEventForm(false)
     },