Add non-intrusive auto-updater

Tauri updater + process plugins. Checks Gitea Releases endpoint a few
seconds after launch; on a newer signed version shows a dismissible
bottom-right banner with download progress and auto-relaunch. Public
signing key in tauri.conf.json; release flow documented in RELEASING.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -558,9 +558,16 @@ async fn p4_depots(state: State<'_, AppState>) -> Result<Vec<Value>, String> {
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
-    tauri::Builder::default()
+    let mut builder = tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
-        .plugin(tauri_plugin_dialog::init())
+        .plugin(tauri_plugin_dialog::init());
+    #[cfg(desktop)]
+    {
+        builder = builder
+            .plugin(tauri_plugin_updater::Builder::new().build())
+            .plugin(tauri_plugin_process::init());
+    }
+    builder
         .manage(AppState::default())
         .invoke_handler(tauri::generate_handler![
             p4_clients,