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-tauri/Cargo.lock

@@ -690,6 +690,7 @@ dependencies = [
  "tauri-plugin-sql",
  "tauri-plugin-stronghold",
  "tauri-plugin-updater",
+ "tauri-plugin-window-state",
 ]
 
 [[package]]
@@ -5617,6 +5618,21 @@ dependencies = [
  "zip 4.6.1",
 ]
 
+[[package]]
+name = "tauri-plugin-window-state"
+version = "2.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "73736611e14142408d15353e21e3cca2f12a3cfb523ad0ce85999b6d2ef1a704"
+dependencies = [
+ "bitflags 2.11.1",
+ "log",
+ "serde",
+ "serde_json",
+ "tauri",
+ "tauri-plugin",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "tauri-runtime"
 version = "2.10.1"