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/resources/views/layouts/knowledge-base.blade.php

@@ -44,7 +44,8 @@
                             <button
                                 type="button"
                                 class="inline-flex items-center px-3 py-2 border border-gray-300 shadow-sm text-sm leading-4 font-medium rounded-md text-gray-700 bg-white hover:bg-gray-50 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-500"
-                                @click.prevent="console.log('Button clicked'); $dispatch('open-quick-switcher')"
+                                x-data
+                                @click.prevent="$dispatch('open-quick-switcher')"
                             >
                                 <svg class="h-4 w-4 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                                     <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"></path>
@@ -76,6 +77,16 @@ class="absolute right-0 mt-2 w-48 bg-white rounded-md shadow-lg py-1 ring-1 ring
                                         <a href="{{ route('profile.edit') }}" class="block px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                                             Profile
                                         </a>
+                                        @if(Auth::user()->isAdmin())
+                                            <a href="{{ route('admin.users.index') }}" class="block px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                                                <span class="flex items-center">
+                                                    <svg class="w-4 h-4 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                                                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4.354a4 4 0 110 5.292M15 21H3v-1a6 6 0 0112 0v1zm0 0h6v-1a6 6 0 00-9-5.197M13 7a4 4 0 11-8 0 4 4 0 018 0z"></path>
+                                                    </svg>
+                                                    ユーザー管理
+                                                </span>
+                                            </a>
+                                        @endif
                                         <form method="POST" action="{{ route('logout') }}">
                                             @csrf
                                             <button type="submit" class="w-full text-left block px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">