Add: Auto-Updater fuer Electron App

- Server: /updates Endpoint fuer Squirrel.Windows Update-Feed
- Electron: autoUpdater mit 30min Check-Intervall
- Preload: IPC-Events fuer Update-Status ans Frontend

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

electron/preload.js

@@ -1,6 +1,9 @@
-const { contextBridge } = require('electron');
+const { contextBridge, ipcRenderer } = require('electron');
 
 contextBridge.exposeInMainWorld('electronAPI', {
   isElectron: true,
   version: '1.5.0',
+  onUpdateAvailable: (callback) => ipcRenderer.on('update-available', callback),
+  onUpdateReady: (callback) => ipcRenderer.on('update-ready', callback),
+  installUpdate: () => ipcRenderer.send('install-update'),
 });