作品展示页：前端展示 + 后台增删改查

- 新增 projects 表与迁移（唯一 slug 索引）
- 后端：公开 GET /api/projects，后台 GET/POST/PUT/DELETE /api/admin/projects
- 前端：/projects 双虚线边框卡片页（参考 diygod.cc/projects），后台管理页
- 左栏/手机顶栏/后台侧栏均加「作品」入口
- 补充项目 CRUD 单元测试与后台鉴权拦截测试

frontend/src/api.js

@@ -54,6 +54,7 @@ export const publicApi = {
   post: (slug) => request('/api/posts/' + encodeURIComponent(slug)),
   archive: () => request('/api/archive'),
   tags: cached(() => request('/api/tags')),
+  projects: cached(() => request('/api/projects')),
   latest: cached(() => request('/api/posts?size=5'))
 }
 
@@ -76,6 +77,10 @@ export const adminApi = {
   deleteTag: (id) => request('/api/admin/tags/' + id, { method: 'DELETE' }),
   mergeTag: (id, toId) =>
     request('/api/admin/tags/' + id + '/merge', { method: 'POST', body: { to_id: toId } }),
+  projects: (params = {}) => request('/api/admin/projects?' + new URLSearchParams(params)),
+  createProject: (body) => request('/api/admin/projects', { method: 'POST', body }),
+  updateProject: (id, body) => request('/api/admin/projects/' + id, { method: 'PUT', body }),
+  deleteProject: (id) => request('/api/admin/projects/' + id, { method: 'DELETE' }),
   settings: () => request('/api/admin/settings'),
   saveSettings: (body) => request('/api/admin/settings', { method: 'PUT', body })
 }