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.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "mail-client",
   "private": true,
-  "version": "0.0.0",
+  "version": "0.1.0",
   "type": "module",
   "scripts": {
     "dev": "vite",
@@ -13,6 +13,8 @@
   "dependencies": {
     "@tauri-apps/api": "^2.11.1",
     "@tauri-apps/plugin-dialog": "^2.7.2",
+    "@tauri-apps/plugin-process": "^2.3.1",
+    "@tauri-apps/plugin-updater": "^2.10.1",
     "clsx": "^2.1.1",
     "date-fns": "^4.4.0",
     "lucide-react": "^1.27.0",