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/Cargo.toml

@@ -23,6 +23,7 @@ tauri-plugin-opener = "2.5.0"
 tauri-plugin-store = "2.4.0"
 tauri-plugin-updater = "2.9.0"
 tauri-plugin-process = "2.3.0"
+tauri-plugin-notification = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 sysinfo = { version = "0.31", default-features = false, features = ["multithread", "network", "system", "disk"] }