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/package.json

@@ -1,7 +1,7 @@
 {
   "name": "gaming-hub-desktop",
   "productName": "Gaming Hub",
-  "version": "1.5.0",
+  "version": "1.5.8",
   "description": "Gaming Hub Desktop App mit Ad-Blocker",
   "author": "Gaming Hub",
   "main": "main.js",