Add auto-updater (Phase 1 #4)

- tauri-plugin-updater + tauri-plugin-process registered (Rust + npm)
- Updater endpoint: github.com/hoornet/wrystr/releases/latest/download/latest.json
- Ed25519 signing keypair generated; public key in tauri.conf.json;
  private key added to TAURI_SIGNING_PRIVATE_KEY GitHub secret
- Release workflow passes TAURI_SIGNING_PRIVATE_KEY env var so
  tauri-action signs artifacts and generates latest.json manifest
- useUpdater hook: checks for updates 5 s after startup (silent on
  error); exposes available, version, install(), dismiss()
- UpdateBanner: dismissible top-of-app bar shown when an update is
  available — "Update & restart" downloads, installs, and relaunches

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -139,6 +139,8 @@ fn db_load_profile(state: tauri::State<DbState>, pubkey: String) -> Result<Optio
 pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
+        .plugin(tauri_plugin_updater::Builder::new().build())
+        .plugin(tauri_plugin_process::init())
         .setup(|app| {
             // ── SQLite ───────────────────────────────────────────────────────
             let data_dir = app.path().app_data_dir()?;