feat: login provider selection, logout, playlists pages

- LoginPage : sélection Discord/GitHub/Google/Twitch via SuperOAuth
- Header : bouton Connexion → /login, logout ↩ quand connecté, nav Playlists conditionnelle
- useAuth : expose setUser pour logout côté Layout
- PlaylistsPage : liste owned/shared, création inline
- PlaylistPage : détail playlist + liste vidéos ordonnées
- Fix : Video.id number → string (UUID)
- Routes : /login, /playlists, /playlists/:id

frontend/src/hooks/useAuth.ts

@@ -2,8 +2,8 @@ import { useState, useEffect } from 'react';
 import { apiFetch } from '../lib/api';
 
 export interface User {
-  id: number;
-  email: string;
+  id: string;
+  email: string | null;
   nickname: string;
   subscriptionLevel?: number;
 }
@@ -11,6 +11,7 @@ export interface User {
 interface AuthState {
   user: User | null;
   loading: boolean;
+  setUser: (u: User | null) => void;
 }
 
 interface MeResponse {
@@ -33,5 +34,5 @@ export function useAuth(): AuthState {
     return () => { cancelled = true; };
   }, []);
 
-  return { user, loading };
+  return { user, loading, setUser };
 }