feat: Admin SynthQuest level management + user dropdown with admin access

SynthQuest admin:
- New "🎮 SynthQuest" section in admin sidebar
- List custom levels with world, ID, title, patch status
- Create new level: world selector, title, subtitle, description,
  concept (hint), available modules (tag input), boss flag, sort order
- Edit existing levels inline
- Import patch base from sandbox JSON export (📥 button per level)
- Delete levels with confirmation

Server:
- custom_levels table (PostgreSQL)
- CRUD API at /api/v1/admin/levels
- POST /:id/import-patch to import sandbox JSON as preplaced modules

Admin access:
- User badge is now a hover dropdown with "🛠 Admin" + "Cerrar sesion"
- Admin visible in Sandbox toolbar, Workshop nav, and user dropdown
- onSwitchToAdmin passed through navigation chain

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/client/src/main.jsx

@@ -20,7 +20,7 @@ function Root() {
 
   return (
     <AuthProvider>
-      {mode === 'sandbox' && <App onSwitchToGame={nav.toGame} onSwitchToWorkshop={nav.toWorkshop} />}
+      {mode === 'sandbox' && <App onSwitchToGame={nav.toGame} onSwitchToWorkshop={nav.toWorkshop} onSwitchToAdmin={nav.toAdmin} />}
       {mode === 'game' && <GameApp onSwitchToSandbox={nav.toSandbox} onSwitchToWorkshop={nav.toWorkshop} />}
       {mode === 'workshop' && <Workshop onSwitchToSandbox={nav.toSandbox} onSwitchToGame={nav.toGame} onSwitchToAdmin={nav.toAdmin} />}
       {mode === 'admin' && <AdminPanel2 onBack={nav.toGame} />}