feat: tray, window-state, sqlite cache, decrypt worker

System tray (desktop)
- tauri tray-icon feature + tray with menu (Öffnen/Ausblenden/Beenden)
- Left-click toggles main window; right-click shows menu
- JS emits tray-unread-update event, Rust mirrors into tooltip +
  macOS dock badge via set_badge_label
- ConversationsContext wires totalUnread → tray

Window state persistence
- tauri-plugin-window-state (desktop-only target guard)
- Auto-restore size/position/maximized between restarts

Local SQLite message cache
- tauri-plugin-sql hydration of conversation view on mount
- persistMessages after each refresh, deleteCachedMessage on realtime
  DELETE, pruneCache keeps latest 1000 per conversation
- Stores plaintext only (same trust boundary as stronghold device
  key; cache never leaves the device, E2EE w.r.t. server unchanged)

Web Worker for decryption
- workers/decrypt.worker.ts runs crypto_secretbox_open_easy + utf-8
  decode off the main thread with its own libsodium instance
- lib/decryptWorker.ts is a request/response wrapper with inline
  fallback when Worker spawn fails
- shared decryptMessages accepts aeadBatchDelegate so key lookup
  stays on the main thread while the AEAD loop offloads

Build fix
- Enable tauri tray-icon feature
- Import Listener + Manager traits, clone tray handle for the
  event listener, conditional icon attach

apps/desktop/src/context/ConversationsContext.tsx

@@ -17,6 +17,7 @@ import {
 import { playNotificationTone } from '../lib/notificationSound';
 import { notify } from '../lib/osNotify';
 import { supabase } from '../lib/supabase';
+import { updateTrayUnread } from '../lib/trayBadge';
 import { useAuth } from './AuthContext';
 
 const LAST_READ_STORAGE_KEY = 'chatapp.conv_last_read';
@@ -253,6 +254,12 @@ export function ConversationsProvider({ children }: { children: ReactNode }) {
     return s;
   }, [unread]);
 
+  // Mirror unread count into the tray tooltip + dock badge. Runs in Tauri
+  // only; no-op in browser-preview.
+  useEffect(() => {
+    void updateTrayUnread(totalUnread);
+  }, [totalUnread]);
+
   const value = useMemo<ConversationsContextValue>(
     () => ({
       conversations,