feat(updater): self-update via GitHub Releases with launch check and settings UI

Phase 3 of the 0.1.0 release prep:
- tauri-plugin-updater + tauri-plugin-process wired into the builder, with
  updater:default / process:allow-restart capabilities
- bundle.createUpdaterArtifacts: true; endpoint points at the GitHub
  releases latest.json, pubkey baked into tauri.conf.json
- store: update status state machine (check / download progress / install)
  with a quiet launch check (prod only) that stays silent on network errors
- UI: dismissible update toast with download progress, plus an Updates group
  in Settings > General showing current version and manual check/install

src-tauri/src/lib.rs

@@ -16,6 +16,8 @@ use tauri::Manager;
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()
+        .plugin(tauri_plugin_updater::Builder::new().build())
+        .plugin(tauri_plugin_process::init())
         .plugin(tauri_plugin_opener::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_fs::init())