feat: voice messages, offline queue, delivery ticks, volume slider, admin + scaling

- Voice messages: MediaRecorder → encrypted attachment, custom waveform
  player via OfflineAudioContext, 60s limit + live mic-level meter
- Offline message queue: localStorage outbox, exponential backoff retries,
  optimistic pending bubble with retry/discard
- Delivery indicator: message_deliveries table + RLS (reciprocal receipts),
  ✓ / ✓✓ / ✓✓-blue tick states, group-aware (all members must ack)
- Per-participant volume slider in calls via right-click tile menu,
  persisted to localStorage, applied to attached audio elements
- Group call scaling: grid up to 12 tiles with pagination,
  active-speaker auto-promotion in fullscreen
- Push notifications scaffolding: service worker, VAPID subscription
  registration, notify-push edge function skeleton
- Backup recovery code: 24-char base32 code (~120 bits entropy) as
  alternative decrypt path, restore UI with mode toggle
- Admin panel: conversations list, audit log (admin_audit_log table +
  admin_log_action RPC), audit entry on user flag toggle
- Search v2: sender filter, attachment-only toggle, date range
- Reactions pop animation (scale 0.4→1.15→1 on count change)
- Message list windowing (150 default, expand via IntersectionObserver)
- Stub cleanup: removed dead ScreenshareStub from CallParticipantTile

Fixes:
- Focus-triggered flicker: dropped window.focus listeners in three spots,
  throttled visibilitychange/online wake-refreshes to 30s, keep existing
  data visible during background re-syncs (no more spinner on every click)
- Voice attachment audio element collapsed to 0px on peer side — now
  forces 280px min-width on bubble

Migrations (push required):
  20260421000001_message_deliveries.sql
  20260421000002_admin_audit_log.sql

Server TODO:
  VAPID keys + notify-push edge function deploy

apps/desktop/src/context/ConversationsContext.tsx

@@ -99,7 +99,9 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
       return;
     }
     try {
-      setLoading(true);
+      // Only flag loading on initial fetch so background re-syncs (visibility
+      // change, online event) don't blank the list each time.
+      setLoading((prev) => (conversationsRef.current.length === 0 ? true : prev));
       const convs = await listConversations(supabase);
       setConversations(convs);
       const entries = await Promise.all(
@@ -218,8 +220,13 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
     // refresh + realtime reconnect on visibility/focus regain so we never
     // leave stale conversation lists on a Windows client after the user
     // returns to the app.
+    let lastAwakeRefresh = 0;
+    const AWAKE_THROTTLE_MS = 30_000;
     const onAwake = () => {
       if (document.visibilityState !== 'visible') return;
+      const now = Date.now();
+      if (now - lastAwakeRefresh < AWAKE_THROTTLE_MS) return;
+      lastAwakeRefresh = now;
       void refresh();
       try {
         // If the socket got wedged during background throttle, a no-op
@@ -231,12 +238,10 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
       }
     };
     document.addEventListener('visibilitychange', onAwake);
-    window.addEventListener('focus', onAwake);
     window.addEventListener('online', onAwake);
 
     return () => {
       document.removeEventListener('visibilitychange', onAwake);
-      window.removeEventListener('focus', onAwake);
       window.removeEventListener('online', onAwake);
       void supabase.removeChannel(channel);
     };