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/lib/updater.ts

@@ -0,0 +1,27 @@
+import { check, type Update } from '@tauri-apps/plugin-updater'
+import { relaunch } from '@tauri-apps/plugin-process'
+
+export async function checkForUpdate(): Promise<Update | null> {
+  try {
+    return await check()
+  } catch {
+    return null
+  }
+}
+
+export async function installUpdate(update: Update, onProgress: (pct: number | null) => void): Promise<void> {
+  let total = 0
+  let downloaded = 0
+  await update.downloadAndInstall((event) => {
+    if (event.event === 'Started') {
+      total = event.data.contentLength ?? 0
+      onProgress(total > 0 ? 0 : null)
+    } else if (event.event === 'Progress') {
+      downloaded += event.data.chunkLength
+      onProgress(total > 0 ? Math.min(100, Math.round((downloaded / total) * 100)) : null)
+    } else if (event.event === 'Finished') {
+      onProgress(100)
+    }
+  })
+  await relaunch()
+}