feat: P1 学习技能包 — 可组合技能模块系统

后端:
- SkillsModule (service + controller)，8 个预置技能
- GET /skills (支持 category/difficulty/search 过滤)
- GET /skills/:id /categories /difficulties

前端:
- /skills 技能市场 — 分类/难度/搜索过滤
- /skills/[id] 技能详情 — system prompt、练习任务、starter
- header 导航新增「技能」入口
- sandbox page 改为从 API 加载技能（替代硬编码 SCENES）
- 支持 ?skill=xxx 直接加载指定技能
- useSearchParams Suspense 包装

全栈: 70 pages / 92 tests 全部通过

frontend/src/i18n/locales/en.ts

@@ -150,6 +150,28 @@ const en: Translations = {
     three: 'Three.js',
     console: 'Console Output',
   },
+  skills: {
+    title: 'Skill Library',
+    desc: 'Composable AI learning skill modules',
+    search: 'Search skills...',
+    allCategories: 'All Categories',
+    allDifficulties: 'All Levels',
+    beginner: 'Beginner',
+    intermediate: 'Intermediate',
+    advanced: 'Advanced',
+    tasks: 'Practice Tasks',
+    starters: 'Try these questions',
+    prerequisites: 'Prerequisites',
+    apply: 'Use this skill',
+    categories: {
+      basic: 'Basic',
+      technical: 'Technical',
+      creative: 'Creative',
+      education: 'Education',
+      advanced: 'Advanced',
+      career: 'Career',
+    },
+  },
   promptWorkshop: {
     title: 'Prompt Workshop',
     desc: 'Write, test, and optimize your prompts',