Add Windows native notifications for chat sessions

- Add tauri-plugin-notification for native Windows notifications
- Send notification when new chat session is started
- Configure notification permissions in capabilities

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/desktop/src-tauri/capabilities/default.json

@@ -13,6 +13,10 @@
     "store:allow-save",
     "store:allow-delete",
     "updater:default",
-    "process:default"
+    "process:default",
+    "notification:default",
+    "notification:allow-notify",
+    "notification:allow-request-permission",
+    "notification:allow-is-permission-granted"
   ]
 }