Fix Electron update: Squirrel feed URL, version display, error details

- Fix: Squirrel.Windows feed URL /updates → /downloads (Squirrel appends /RELEASES)
- Show Desktop App + Server version in update modal
- Display actual error message in failed state
- Dynamic Electron version via app.getVersion() instead of hardcoded
- Sync electron/package.json version with VERSION file (1.5.8)
- Add "Später" button on update-ready, timeout error message
- Style: version info, error detail box, secondary button

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

electron/preload.js

@@ -2,7 +2,8 @@ const { contextBridge, ipcRenderer } = require('electron');
 
 contextBridge.exposeInMainWorld('electronAPI', {
   isElectron: true,
-  version: '1.5.0',
+  // Version comes from main process (reads package.json via app.getVersion())
+  version: ipcRenderer.sendSync('get-app-version'),
   onUpdateAvailable: (callback) => ipcRenderer.on('update-available', callback),
   onUpdateReady: (callback) => ipcRenderer.on('update-ready', callback),
   onUpdateNotAvailable: (callback) => ipcRenderer.on('update-not-available', callback),