fix: ApiError typée + error handling pages video/playlists/admin

- api.ts : ApiError class (status: number) — remplace Error générique
- VideoPage/PlaylistPage : instanceof ApiError au lieu de message.includes()
- PlaylistsPage : fetchError + createError — silent catch supprimé
- AdminPage : guard roles.some() aligné Header (super_admin inclus)

frontend/src/pages/AdminPage.tsx

@@ -50,7 +50,7 @@ export default function AdminPage() {
   const [tab, setTab] = useState<Tab>('videos');
 
   if (authLoading) return null;
-  if (!user?.roles?.includes('admin')) return <Navigate to="/" replace />;
+  if (!user?.roles?.some((r) => r === 'admin' || r === 'super_admin')) return <Navigate to="/" replace />;
 
   return (
     <div className="flex flex-col gap-6">