Refactor: Zentralisiertes Admin-Login im Top-Menü

- Admin-Login aus 3 Plugins (Soundboard, Streaming, Game Library) entfernt
- Zentraler 🔒/🔓 Button im Header mit Login-Modal
- isAdmin wird als Prop an alle Plugins weitergegeben
- Settings-Buttons (Gear-Icons) nur sichtbar wenn eingeloggt
- Alle Plugins nutzen weiterhin den shared admin-Cookie für Operationen
- Login/Logout-Formulare und Buttons aus Plugin-Panels entfernt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/styles.css

@@ -353,6 +353,104 @@ html, body {
   background: rgba(230, 126, 34, 0.1);
 }
 
+/* ── Admin Button (header) ── */
+.hub-admin-btn {
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  color: var(--text-muted);
+  font-size: 16px;
+  padding: 4px 8px;
+  cursor: pointer;
+  transition: all var(--transition);
+  line-height: 1;
+}
+.hub-admin-btn:hover {
+  color: var(--accent);
+  border-color: var(--accent);
+}
+.hub-admin-btn.active {
+  color: #4ade80;
+  border-color: #4ade80;
+}
+
+/* ── Admin Login Modal ── */
+.hub-admin-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 9999;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(0, 0, 0, 0.7);
+  backdrop-filter: blur(4px);
+}
+.hub-admin-modal {
+  background: var(--bg-card);
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  width: 340px;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5);
+}
+.hub-admin-modal-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 16px 20px;
+  border-bottom: 1px solid var(--border);
+  font-weight: 600;
+}
+.hub-admin-modal-close {
+  background: none;
+  border: none;
+  color: var(--text-muted);
+  cursor: pointer;
+  font-size: 16px;
+}
+.hub-admin-modal-close:hover {
+  color: var(--text);
+}
+.hub-admin-modal-body {
+  padding: 20px;
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+}
+.hub-admin-input {
+  width: 100%;
+  padding: 8px 12px;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  background: var(--bg-secondary);
+  color: var(--text);
+  font-size: 14px;
+  font-family: var(--font);
+  box-sizing: border-box;
+}
+.hub-admin-input:focus {
+  outline: none;
+  border-color: var(--accent);
+}
+.hub-admin-error {
+  color: #ef4444;
+  font-size: 13px;
+  margin: 0;
+}
+.hub-admin-submit {
+  padding: 8px 16px;
+  background: var(--accent);
+  color: #fff;
+  border: none;
+  border-radius: var(--radius);
+  font-size: 14px;
+  font-weight: 500;
+  cursor: pointer;
+  transition: opacity var(--transition);
+}
+.hub-admin-submit:hover {
+  opacity: 0.9;
+}
+
 /* ── Version Info Modal ── */
 .hub-version-clickable {
   cursor: pointer;