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-lock.json

@@ -10,6 +10,8 @@
       "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",
@@ -1500,6 +1502,24 @@
         "@tauri-apps/api": "^2.11.0"
       }
     },
+    "node_modules/@tauri-apps/plugin-process": {
+      "version": "2.3.1",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-process/-/plugin-process-2.3.1.tgz",
+      "integrity": "sha512-nCa4fGVaDL/B9ai03VyPOjfAHRHSBz5v6F/ObsB73r/dA3MHHhZtldaDMIc0V/pnUw9ehzr2iEG+XkSEyC0JJA==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.8.0"
+      }
+    },
+    "node_modules/@tauri-apps/plugin-updater": {
+      "version": "2.10.1",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-updater/-/plugin-updater-2.10.1.tgz",
+      "integrity": "sha512-NFYMg+tWOZPJdzE/PpFj2qfqwAWwNS3kXrb1tm1gnBJ9mYzZ4WDRrwy8udzWoAnfGCHLuePNLY1WVCNHnh3eRA==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.10.1"
+      }
+    },
     "node_modules/@tybys/wasm-util": {
       "version": "0.10.3",
       "resolved": "https://registry.npmjs.org/@tybys/wasm-util/-/wasm-util-0.10.3.tgz",