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>

web/src/styles.css

@@ -313,6 +313,28 @@ html, body {
 .hub-update-btn:hover {
   opacity: 0.85;
 }
+.hub-update-btn-secondary {
+  background: var(--bg-tertiary);
+  color: var(--text-secondary);
+  margin-top: 4px;
+}
+.hub-update-versions {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  margin: 8px 0;
+  font-size: 12px;
+  color: var(--text-muted);
+}
+.hub-update-error-detail {
+  font-size: 11px;
+  color: #ef4444;
+  background: rgba(239, 68, 68, 0.1);
+  border-radius: var(--radius);
+  padding: 6px 10px;
+  word-break: break-word;
+  max-width: 300px;
+}
 
 /* ── Main Content Area ── */
 .hub-content {