feat(timer): wire VisualTimer to tasks with notifications and persistence

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -26,6 +26,7 @@ tauri = { version = "2", features = ["tray-icon"] }
 tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-global-shortcut = "2"
+tauri-plugin-notification = "2"
 
 # Serialisation
 serde = { version = "1", features = ["derive"] }