Add: Check-for-Updates Button in Electron App

- Manueller Update-Check per Button im Header
- Modal-Zustaende: checking, downloading, ready, uptodate, error
- IPC: check-for-updates, update-not-available Events

web/src/styles.css

@@ -226,6 +226,27 @@ html, body {
   border-radius: 4px;
 }
 
+/* ── Check for Updates Button ── */
+.hub-check-update-btn {
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  color: var(--text-secondary);
+  font-size: 14px;
+  padding: 2px 8px;
+  cursor: pointer;
+  transition: all var(--transition);
+  line-height: 1;
+}
+.hub-check-update-btn:hover:not(:disabled) {
+  color: var(--accent);
+  border-color: var(--accent);
+}
+.hub-check-update-btn:disabled {
+  opacity: 0.4;
+  cursor: default;
+}
+
 /* ── Update Modal ── */
 .hub-update-overlay {
   position: fixed;