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/capabilities/default.json

@@ -12,6 +12,8 @@
     "notification:default",
     "core:webview:allow-create-webview-window",
     "core:window:allow-close",
-    "core:window:allow-set-focus"
+    "core:window:allow-set-focus",
+    "updater:default",
+    "process:default"
   ]
 }