添加管理后台公告管理功能

## 新增功能
- 管理后台公告管理页面（/admin/announcements）
- 公告 CRUD 操作（创建、编辑、删除）
- 公告发布和归档功能
- 按状态和分类筛选
- 在侧边栏添加公告管理入口

## 功能特性
- 支持设置优先级、置顶、重要标记
- 富文本编辑器（支持 Markdown/HTML）
- 实时查看浏览次数
- 权限控制（announcement:view 和 announcement:manage）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/features/admin/api/adminAnnouncements.ts

@@ -0,0 +1,58 @@
+import { apiClient } from '@/shared/api/client'
+import type { ApiResponse } from '@/shared/types/types'
+import type { Announcement, PaginatedResult } from './announcements'
+
+export interface CreateAnnouncementRequest {
+  title: string
+  content: string
+  category: 'notice' | 'tutorial' | 'rule' | 'faq'
+  priority?: number
+  is_pinned?: boolean
+  is_important?: boolean
+}
+
+export interface UpdateAnnouncementRequest {
+  title?: string
+  content?: string
+  category?: 'notice' | 'tutorial' | 'rule' | 'faq'
+  priority?: number
+  is_pinned?: boolean
+  is_important?: boolean
+}
+
+export async function fetchAdminAnnouncements(params: {
+  status?: string
+  category?: string
+  page?: number
+  page_size?: number
+}): Promise<PaginatedResult<Announcement>> {
+  const { data } = await apiClient.get<ApiResponse<PaginatedResult<Announcement>>>('/admin/announcements', { params })
+  return data.data
+}
+
+export async function fetchAdminAnnouncementDetail(id: number): Promise<Announcement> {
+  const { data } = await apiClient.get<ApiResponse<Announcement>>(`/admin/announcements/${id}`)
+  return data.data
+}
+
+export async function createAnnouncement(req: CreateAnnouncementRequest): Promise<Announcement> {
+  const { data } = await apiClient.post<ApiResponse<Announcement>>('/admin/announcements', req)
+  return data.data
+}
+
+export async function updateAnnouncement(id: number, req: UpdateAnnouncementRequest): Promise<Announcement> {
+  const { data } = await apiClient.put<ApiResponse<Announcement>>(`/admin/announcements/${id}`, req)
+  return data.data
+}
+
+export async function publishAnnouncement(id: number): Promise<void> {
+  await apiClient.post(`/admin/announcements/${id}/publish`)
+}
+
+export async function archiveAnnouncement(id: number): Promise<void> {
+  await apiClient.post(`/admin/announcements/${id}/archive`)
+}
+
+export async function deleteAnnouncement(id: number): Promise<void> {
+  await apiClient.delete(`/admin/announcements/${id}`)
+}