前端极简黑白重设计,并纳入客户端注册、站点设置与后台相关改动

- storefront 全站改为极简黑白配色:直角、细线分隔、卡片 hover 浅底、深色模式(跟随系统+手动切换)
- 抽出 storefront/partials/card 复用卡片
- 一并提交此前暂存的客户端注册 API、站点设置、后台页面与测试

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Models/StoreSetting.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class StoreSetting extends Model
+{
+    protected $fillable = [
+        'key',
+        'value',
+    ];
+}