Add: Check-for-Updates Button in Electron App

- Manueller Update-Check per Button im Header
- Modal-Zustaende: checking, downloading, ready, uptodate, error
- IPC: check-for-updates, update-not-available Events

electron/main.js

@@ -39,6 +39,12 @@ function setupAutoUpdater() {
 
   autoUpdater.on('update-not-available', () => {
     console.log('[AutoUpdater] App is up to date.');
+    if (mainWindow) mainWindow.webContents.send('update-not-available');
+  });
+
+  // Manual check from renderer
+  ipcMain.on('check-for-updates', () => {
+    try { autoUpdater.checkForUpdates(); } catch (e) { console.error('[AutoUpdater]', e.message); }
   });
 
   autoUpdater.on('error', (err) => {