Convert AdminPanel to modal overlay with close button and add admin dashboard menu option

Refactored AdminPanel from inline component to modal overlay triggered from user menu. Added `open` and `onClose` props to control visibility, wrapped panel in modal overlay with backdrop click-to-close behavior, and added close button to header. Implemented admin-only menu item in TopBar user dropdown with shield icon and role label. Added state management for dashboard visibility in App.jsx with proper cleanup

frontend/src/App.jsx

@@ -61,6 +61,7 @@ export default function App() {
   const [chipOpen, setChipOpen] = useState(false);
   const [chipEntry, setChipEntry] = useState(null);
   const [userMenuOpen, setUserMenuOpen] = useState(false);
+  const [adminOpen, setAdminOpen] = useState(false);
 
   const [pwOpen, setPwOpen] = useState(false);
   const [pw1, setPw1] = useState("");
@@ -689,11 +690,15 @@ export default function App() {
           openPwModal={openPwModal}
           openDesignModal={openDesignModal}
           openStatsModal={openStatsModal}
+          openAdminPanel={() => {
+            setAdminOpen(true);
+            setUserMenuOpen(false);
+          }}
           doLogout={doLogout}
           onOpenNewGame={() => setNewGameOpen(true)}
         />
 
-        {me.role === "admin" && <AdminPanel />}
+        <AdminPanel open={me.role === "admin" && adminOpen} onClose={() => setAdminOpen(false)} />
 
         <GamePickerCard
           games={games}