Add admin user management and improve UX

Features:
- Add user management for admins (CRUD operations)
- Add is_admin column to users table
- Add AdminMiddleware for authorization
- Add admin routes and views
- Add artisan command: user:set-admin

Improvements:
- Fix Quick Switcher: add x-data for Alpine.js dispatch
- Fix Quick Switcher: close on outside click
- Redirect non-admin users to front page after login
- Add Knowledge Base link in dashboard navigation
- Change app logo from Laravel to book icon

src/app/Models/User.php

@@ -21,8 +21,19 @@ class User extends Authenticatable
         'name',
         'email',
         'password',
+        'is_admin',
     ];
 
+    /**
+     * Check if user is an administrator.
+     *
+     * @return bool
+     */
+    public function isAdmin(): bool
+    {
+        return (bool) $this->is_admin;
+    }
+
     /**
      * The attributes that should be hidden for serialization.
      *