feat: 技能库改为数据库存储 (Prisma Skill 模型 + seed)

- Prisma 新增 Skill 模型 (id/name/description/icon/category/difficulty/systemPrompt/starters/tasks/tags/prerequisiteIds)
- SkillsService 从 DB 读取替代硬编码常量
- seed.ts 新增 seedSkills() 写入 8 个技能
- SkillsController 保持不变，API 返回 JSON 解析后的 starters/tasks

frontend/src/app/sandbox/page.tsx

@@ -292,7 +292,16 @@ function SandboxPage() {
     }).then(() => alert('分享成功！')).catch(() => alert('分享失败'));
   }
 
-  const currentScene = SCENES.find(s => s.id === scene) || SCENES[0];
+  const currentScene = SCENES.find(s => s.id === scene) || SCENES[0] || null;
+
+  if (skillsLoading) return (
+    <div className="max-w-6xl mx-auto px-4 sm:px-6 lg:px-8 py-8">
+      <div className="animate-pulse space-y-4">
+        <div className="h-8 w-48 bg-muted rounded-lg" />
+        <div className="h-[65vh] bg-muted rounded-2xl" />
+      </div>
+    </div>
+  );
   const isNewChat = messages.length <= 1 && messages[0]?.role === 'assistant';
 
   return (