Add auto-update mechanism via tauri-plugin-updater

Users must manually check for and download new versions. Tauri 2.x supports `tauri-plugin-updater` for seamless auto-updates.

**Required:**
- Add `tauri-plugin-updater` dependency
- Configure update endpoint (e.g., GitHub Releases)
- Generate and manage update signing keys
- Add update check UI in the app (or silent background check with notification)

Start by reviewing the Tauri 2.x configuration and release setup, then read the tauri-plugin-updater documentation and its GitHub Releases endpoint requirements. Done means the dependency is configured, signing keys are managed, and the app checks for updates either through UI or a background notification.

Written by the indexing model from the issue text.