Add server control buttons to dashboard with confirmation dialogs

- Add ConfirmModal component for stop/restart confirmations
- Add start/stop/restart buttons to ServerCard (moderator/admin only)
- Add confirmation dialogs to ServerDetail for stop/restart actions
- Add btn-sm CSS class for smaller buttons

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gsm-frontend/src/index.css

@@ -52,6 +52,11 @@ button {
   cursor: not-allowed;
 }
 
+.btn-sm {
+  font-size: 0.75rem;
+  padding: 0.375rem 0.75rem;
+}
+
 .btn-primary {
   background-color: #fafafa;
   color: #0a0a0a;