feat: reorganize Filament into multi-panel structure and add is_admin cast

- Move Filament resources, pages, and widgets into Admin namespace subdirectories
- Remove default panel and login method calls from AdminPanelProvider
- Add is_admin boolean cast to User model

app/Models/User.php

@@ -31,6 +31,7 @@ class User extends Authenticatable implements FilamentUser
         return [
             'email_verified_at' => 'datetime',
             'password' => 'hashed',
+            'is_admin' => 'boolean',
         ];
     }