feat: 實作後台導航選單系統與狀態持久化

- 新增 14 個模組的路由與控制器佔位符
- 實作可展開式側邊欄選單 (Sidebar Menu)
- 優化選單樣式與主題適配
- 實作選單展開狀態持久化 (LocalStorage)
- 修復子選單縮排與顏色問題

resources/views/admin/placeholder.blade.php

@@ -0,0 +1,47 @@
+@php
+    $theme = request()->cookie('theme', 'dark-blue');
+    $themes = [
+        'dark-blue' => ['card' => 'bg-gray-800', 'accent' => 'indigo'],
+        'dark-purple' => ['card' => 'bg-slate-800', 'accent' => 'purple'],
+        'dark-green' => ['card' => 'bg-zinc-800', 'accent' => 'emerald'],
+        'light-blue' => ['card' => 'bg-white', 'accent' => 'blue'],
+        'light-green' => ['card' => 'bg-white', 'accent' => 'green'],
+    ];
+    $currentTheme = $themes[$theme] ?? $themes['dark-blue'];
+    $isLight = in_array($theme, ['light-blue', 'light-green']);
+@endphp
+
+@extends('layouts.admin')
+
+@section('content')
+<div class="max-w-7xl mx-auto">
+    <div class="{{ $currentTheme['card'] }} rounded-lg shadow-lg p-8 text-center border {{ $isLight ? 'border-gray-200' : 'border-gray-700' }}">
+        <div class="mb-6">
+            <svg class="mx-auto h-24 w-24 text-{{ $currentTheme['accent'] }}-500" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 6V4m0 2a2 2 0 100 4m0-4a2 2 0 110 4m-6 8a2 2 0 100-4m0 4a2 2 0 110-4m0 4v2m0-6V4m6 6v10m6-2a2 2 0 100-4m0 4a2 2 0 110-4m0 4v2m0-6V4" />
+            </svg>
+        </div>
+        <h1 class="text-3xl font-bold {{ $isLight ? 'text-gray-900' : 'text-white' }} mb-4">{{ $title ?? '功能頁面' }}</h1>
+        <p class="{{ $isLight ? 'text-gray-600' : 'text-gray-400' }} mb-6 text-lg">{{ $description ?? '此功能正在開發中' }}</p>
+        <div class="inline-block px-6 py-3 bg-{{ $currentTheme['accent'] }}-600 text-white rounded-lg font-semibold">
+            🚧 功能開發中
+        </div>
+        
+        @if(isset($features) && count($features) > 0)
+        <div class="mt-8 text-left max-w-2xl mx-auto">
+            <h3 class="text-xl font-semibold {{ $isLight ? 'text-gray-900' : 'text-white' }} mb-4">規劃功能：</h3>
+            <ul class="space-y-2 {{ $isLight ? 'text-gray-700' : 'text-gray-300' }}">
+                @foreach($features as $feature)
+                <li class="flex items-start">
+                    <svg class="h-6 w-6 text-{{ $currentTheme['accent'] }}-500 mr-2 flex-shrink-0" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 12l2 2 4-4m6 2a9 9 0 11-18 0 9 9 0 0118 0z" />
+                    </svg>
+                    <span>{{ $feature }}</span>
+                </li>
+                @endforeach
+            </ul>
+        </div>
+        @endif
+    </div>
+</div>
+@endsection