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/App.tsx

@@ -28,6 +28,7 @@ export default function App() {
   const [connected, setConnected] = useState(false);
   const [plugins, setPlugins] = useState<PluginInfo[]>([]);
   const [updateState, setUpdateState] = useState<'idle' | 'checking' | 'downloading' | 'ready' | 'uptodate' | 'error'>('idle');
+  const [updateError, setUpdateError] = useState<string>('');
   const [activeTab, setActiveTabRaw] = useState<string>(() => localStorage.getItem('hub_activeTab') ?? '');
 
   const setActiveTab = (tab: string) => {
@@ -118,15 +119,22 @@ export default function App() {
       }
     });
     api.onUpdateNotAvailable?.(() => setUpdateState('uptodate'));
-    api.onUpdateError?.(() => setUpdateState('error'));
+    api.onUpdateError?.((msg: string) => {
+      setUpdateError(msg || 'Unbekannter Fehler');
+      setUpdateState('error');
+    });
   }, []);
 
+  const electronVersion = (window as any).electronAPI?.version ?? null;
+
   const handleCheckForUpdates = () => {
     setUpdateState('checking');
+    setUpdateError('');
     (window as any).electronAPI?.checkForUpdates?.();
     // Timeout: falls Electron nicht antwortet, nach 15s zurücksetzen
     setTimeout(() => {
       setUpdateState(prev => prev === 'checking' ? 'error' : prev);
+      setUpdateError(prev => prev || 'Zeitüberschreitung — Server nicht erreichbar.');
     }, 15000);
   };
 
@@ -199,6 +207,10 @@ export default function App() {
               <>
                 <div className="hub-update-icon">{'\u{1F50D}'}</div>
                 <h2>Suche nach Updates...</h2>
+                <div className="hub-update-versions">
+                  {electronVersion && <span>Desktop App: v{electronVersion}</span>}
+                  <span>Server: v{version}</span>
+                </div>
                 <div className="hub-update-progress">
                   <div className="hub-update-progress-bar" />
                 </div>
@@ -210,8 +222,10 @@ export default function App() {
             {updateState === 'downloading' && (
               <>
                 <div className="hub-update-icon">{'\u2B07\uFE0F'}</div>
-                <h2>Update verfügbar</h2>
-                <p>Update wird heruntergeladen...</p>
+                <h2>Update wird heruntergeladen...</h2>
+                <div className="hub-update-versions">
+                  {electronVersion && <span>Aktuell: v{electronVersion}</span>}
+                </div>
                 <div className="hub-update-progress">
                   <div className="hub-update-progress-bar" />
                 </div>
@@ -223,7 +237,10 @@ export default function App() {
                 <h2>Update bereit!</h2>
                 <p>Die App wird neu gestartet, um das Update zu installieren.</p>
                 <button className="hub-update-btn" onClick={() => (window as any).electronAPI?.installUpdate()}>
-                  OK
+                  Jetzt installieren
+                </button>
+                <button className="hub-update-btn hub-update-btn-secondary" onClick={() => setUpdateState('idle')}>
+                  Später
                 </button>
               </>
             )}
@@ -231,7 +248,11 @@ export default function App() {
               <>
                 <div className="hub-update-icon">{'\u2705'}</div>
                 <h2>Alles aktuell!</h2>
-                <p>Du verwendest bereits die neueste Version (v{version}).</p>
+                <div className="hub-update-versions">
+                  {electronVersion && <span>Desktop App: v{electronVersion}</span>}
+                  <span>Server: v{version}</span>
+                </div>
+                <p>Du verwendest bereits die neueste Version.</p>
                 <button className="hub-update-btn" onClick={() => setUpdateState('idle')}>
                   OK
                 </button>
@@ -241,7 +262,11 @@ export default function App() {
               <>
                 <div className="hub-update-icon">{'\u274C'}</div>
                 <h2>Update fehlgeschlagen</h2>
-                <p>Das Update konnte nicht heruntergeladen werden.</p>
+                <div className="hub-update-versions">
+                  {electronVersion && <span>Desktop App: v{electronVersion}</span>}
+                  <span>Server: v{version}</span>
+                </div>
+                {updateError && <p className="hub-update-error-detail">{updateError}</p>}
                 <button className="hub-update-btn" onClick={() => setUpdateState('idle')}>
                   Schließen
                 </button>

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 {