fix: avatar persistant — correction référence template + avatar dropdown

- settings.html: avatarUrl lisait user.avatar_url (dict session, inexistant)
  → corrigé en {{ avatar_url }} (variable template top-level de _sidebar_data)
- settings.html: avatarColor lisait user.get("avatar_color") (idem)
  → corrigé en {{ avatar_color }}
- base.html: dropdown user-menu affiche maintenant l'avatar avec couleur/URL
  au lieu du workspaceInitial statique (pas de synchronisation)

app/templates/base.html

@@ -75,7 +75,7 @@
     </div>
     <div class="user-menu-dropdown" x-show="workspaceMenuOpen" @click.away="workspaceMenuOpen = false" x-transition>
       <div class="um-user-info">
-        <div class="um-avatar">{{ workspaceInitial }}</div>
+        <div class="um-avatar" style="{% if avatar_url %}background-image:url('{{ avatar_url }}');background-size:cover;{% else %}background:{{ avatar_color }};{% endif %}">{% if not avatar_url %}{{ user.full_name[:1].upper() if user and user.get('full_name') else workspaceInitial }}{% endif %}</div>
         <div>
           <div class="um-name">{{ user.full_name or user.login }}</div>
           <div class="um-plan">{{ user.email or '' }}</div>