feat(06-05): global session-expiry interstitial via QueryCache/MutationCache onError

- Add sessionExpired boolean + setSessionExpired action to Zustand calendarStore
- main.tsx: construct QueryClient with QueryCache+MutationCache onError (v5 pattern, not defaultOptions.onError)
- onGlobalError: checks instanceof SessionExpiredError, calls setSessionExpired(true) via store.getState()
- CalendarShell: read sessionExpired from store; render AuthSplash(state=redirecting, 'Session expired', 'Signing you back in…')
- CalendarShell: useEffect fires clearLoginRedirect + maybeRedirectToLogin after 1.5s when sessionExpired (one-shot guard re-armed)
- Context7 /tanstack/query confirmed v5 QueryCache/MutationCache constructor + onError signature

apps/pwa/src/store/calendarStore.ts

@@ -50,6 +50,12 @@ export interface CalendarStore {
   /** UID of the most recently enqueued write. SyncStateToast polls sync-status for this. */
   lastSyncedUid: string | null
 
+  // ── Session-expiry flag (Plan 06-05 / D-11) ─────────────────────────────
+  // Set to true by the global QueryCache/MutationCache onError handler in main.tsx
+  // when a SessionExpiredError is detected from any query or mutation.
+  // Drives the "Session expired / Signing you back in…" interstitial in CalendarShell.
+  sessionExpired: boolean
+
   setSelectedView: (view: string) => void
   setSelectedDate: (date: string) => void
   setOpenEventId: (id: string | null) => void
@@ -77,6 +83,14 @@ export interface CalendarStore {
    * Pass null to dismiss the toast.
    */
   setLastSyncedUid: (uid: string | null) => void
+
+  /**
+   * Arm the session-expiry interstitial (D-11).
+   * Called imperatively from the global QueryCache/MutationCache onError handler
+   * (outside React, via getState().setSessionExpired) when a SessionExpiredError
+   * is caught from any query or mutation.
+   */
+  setSessionExpired: (expired: boolean) => void
 }
 
 // ── Helpers ────────────────────────────────────────────────────────────────
@@ -141,6 +155,9 @@ export const useCalendarStore = create<CalendarStore>((set) => ({
   deleteDialogUid: null,
   lastSyncedUid: null,
 
+  // Session-expiry flag — false by default; set by global QueryCache/MutationCache handler
+  sessionExpired: false,
+
   setSelectedView: (view: string) => {
     set({ selectedView: view })
     // Persist to localStorage keyed by breakpoint group
@@ -166,4 +183,6 @@ export const useCalendarStore = create<CalendarStore>((set) => ({
     set({ deleteDialogOpen: open, deleteDialogUid: uid }),
 
   setLastSyncedUid: (uid: string | null) => set({ lastSyncedUid: uid }),
+
+  setSessionExpired: (expired: boolean) => set({ sessionExpired: expired }),
 }))