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/tauri.conf.json

@@ -28,6 +28,7 @@
   "bundle": {
     "active": true,
     "targets": "all",
+    "createUpdaterArtifacts": true,
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",
@@ -38,5 +39,13 @@
     "android": {
       "debugApplicationIdSuffix": ".debug"
     }
+  },
+  "plugins": {
+    "updater": {
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDZCQTc0QThDNzI0RURGODAKUldTQTMwNXlqRXFuYXdsbm5nMUhBMVpzUVRxdTlzcDUvS3g1SGdHSjFPNkh3cW1rSUN5MVlyMkQK",
+      "endpoints": [
+        "https://git.kanjiv.at/KanjiG/mail-client/releases/download/latest-manifest/latest.json"
+      ]
+    }
   }
 }