feat: chatUsageBadge-Setting + Dashboard-Menü als Dropdown

- Neues Setting chatUsageBadge (default: false) blendet Usage-Badge im Chat ein/aus
- Dashboard-Links durch 3-Dot-Header-Menü ersetzt (einheitlich mit Chat)
- Admin-Items über admin-only Attribut gesteuert, dashboard-links CSS entfernt

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

wwwroot/css/dashboard.css

@@ -2,38 +2,7 @@
   padding: var(--page-padding);
 }
 
-.dashboard header {
-  overflow: auto;
-}
 
-.dashboard header * {
-  white-space: nowrap;
-}
-
-.dashboard-links {
-  display: flex;
-  gap: var(--box-spacing);
-  align-items: center;
-  flex: 1;
-}
-
-.dashboard-link {
-  color: var(--text);
-  text-decoration: none;
-  opacity: var(--semi-transparent);
-  transition: opacity var(--transition-duration);
-  cursor: pointer;
-}
-
-.dashboard-link:hover {
-  opacity: 1;
-  color: var(--text-accent);
-}
-
-.dashboard-link:last-child {
-  margin-left: auto;
-  margin-right: var(--badge-spacing);
-}
 
 .dashboard section {
   margin-bottom: var(--page-spacing);