Add auto-update support (tauri-plugin-updater) and Gitea release pipeline

Ed25519-signed updates, checked via a fixed-tag latest.json manifest on
Gitea Releases; scripts/release.sh automates build+sign+publish. Adds
a manual "Nach Updates suchen" trigger plus a silent startup check.

src-tauri/src/lib.rs

@@ -27,6 +27,8 @@ pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_notification::init())
+        .plugin(tauri_plugin_process::init())
+        .plugin(tauri_plugin_updater::Builder::new().build())
         .setup(|app| {
             if cfg!(debug_assertions) {
                 app.handle().plugin(