feat: Phase 3 Desktop — native menu, auto-update, hidden terminal

- #![windows_subsystem = "windows"]: no console window on Windows release builds
- CREATE_NO_WINDOW (0x08000000): Python backend subprocess hidden on Windows
- Native menu bar: Fichier (N/Fermer/Quitter) + Edition (Undo/Redo/Cut/Copy/Paste/SelectAll) + Aide (About)
- Auto-update: tauri-plugin-updater checks Gitea releases endpoint
- tauri.conf.json: updater plugin config with passive install mode
- ROADMAP: Phase 3 marked as complete (11/11 subtasks done)

desktop/Cargo.toml

@@ -17,6 +17,7 @@ tauri-plugin-fs = "2"
 tauri-plugin-process = "2"
 tauri-plugin-store = "2"
 tauri-plugin-single-instance = "2"
+tauri-plugin-updater = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 reqwest = { version = "0.12", features = ["json"] }