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/PlaylistPage.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react';
 import { useParams, Link } from 'react-router-dom';
-import { apiFetch } from '../lib/api';
+import { apiFetch, ApiError } from '../lib/api';
 
 interface Video {
   id: string;
@@ -35,8 +35,8 @@ export default function PlaylistPage() {
     if (!id) return;
     apiFetch<PlaylistResponse>(`/playlists/${id}`)
       .then((res) => setData(res.data))
-      .catch((err: Error) => {
-        if (err.message.includes('403')) setError('forbidden');
+      .catch((err: unknown) => {
+        if (err instanceof ApiError && err.status === 403) setError('forbidden');
         else setError('not_found');
       })
       .finally(() => setLoading(false));