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/main.js

@@ -10,11 +10,19 @@ try {
 }
 
 const HUB_URL = process.env.GAMING_HUB_URL || 'https://hub.daddelolymp.de';
+const APP_VERSION = app.getVersion();
+
+// Sync IPC: preload reads app version from package.json
+ipcMain.on('get-app-version', (event) => {
+  event.returnValue = APP_VERSION;
+});
 
 function setupAutoUpdater() {
   if (process.platform !== 'win32') return;
 
-  const updateURL = `${HUB_URL}/updates`;
+  // Squirrel.Windows appends /RELEASES to the feed URL,
+  // so point to /downloads where RELEASES + nupkg files live
+  const updateURL = `${HUB_URL}/downloads`;
   try {
     autoUpdater.setFeedURL({ url: updateURL });
   } catch (e) {
@@ -93,7 +101,7 @@ function createWindow() {
 
   // Custom User-Agent to identify Electron app
   const currentUA = mainWindow.webContents.getUserAgent();
-  mainWindow.webContents.setUserAgent(currentUA + ' GamingHubDesktop/1.5.0');
+  mainWindow.webContents.setUserAgent(currentUA + ` GamingHubDesktop/${APP_VERSION}`);
 
   mainWindow.loadURL(HUB_URL);