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/routes/web.php

@@ -1,6 +1,7 @@
 <?php
 
 use App\Http\Controllers\ProfileController;
+use App\Http\Controllers\Admin\UserController as AdminUserController;
 use App\Livewire\DocumentViewer;
 use App\Livewire\DocumentEditor;
 use App\Models\Document;
@@ -26,6 +27,11 @@
     Route::delete('/profile', [ProfileController::class, 'destroy'])->name('profile.destroy');
 });
 
+// Admin routes
+Route::middleware(['auth', 'admin'])->prefix('admin')->name('admin.')->group(function () {
+    Route::resource('users', AdminUserController::class)->except(['show']);
+});
+
 Route::prefix('documents')->name('documents.')->group(function () {
     // 認証が必要なルート（より具体的なルートを先に定義）
     Route::middleware('auth')->group(function () {