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/components/AnnouncementDialog.vue

@@ -0,0 +1,169 @@
+<script setup lang="ts">
+import { computed, ref, watch } from 'vue'
+import type { FormInstance, FormRules } from 'element-plus'
+import { Bell, Document, QuestionFilled, Warning } from '@element-plus/icons-vue'
+import type { Announcement } from '@/features/announcement'
+import type { CreateAnnouncementRequest, UpdateAnnouncementRequest } from '@/features/admin/api/adminAnnouncements'
+
+interface Props {
+  visible: boolean
+  mode: 'create' | 'edit'
+  announcement?: Announcement | null
+}
+
+const props = defineProps<Props>()
+const emit = defineEmits<{
+  'update:visible': [value: boolean]
+  save: [data: CreateAnnouncementRequest | UpdateAnnouncementRequest]
+}>()
+
+const formRef = ref<FormInstance>()
+const formData = ref<CreateAnnouncementRequest>({
+  title: '',
+  content: '',
+  category: 'notice',
+  priority: 0,
+  is_pinned: false,
+  is_important: false,
+})
+
+const categories = [
+  { value: 'notice', label: '通知公告', icon: Bell },
+  { value: 'tutorial', label: '使用教程', icon: Document },
+  { value: 'rule', label: '规则说明', icon: Warning },
+  { value: 'faq', label: '常见问题', icon: QuestionFilled },
+]
+
+const rules: FormRules = {
+  title: [
+    { required: true, message: '请输入公告标题', trigger: 'blur' },
+    { max: 255, message: '标题不能超过255个字符', trigger: 'blur' },
+  ],
+  content: [
+    { required: true, message: '请输入公告内容', trigger: 'blur' },
+  ],
+  category: [
+    { required: true, message: '请选择公告分类', trigger: 'change' },
+  ],
+}
+
+const dialogTitle = computed(() => {
+  return props.mode === 'create' ? '新建公告' : '编辑公告'
+})
+
+watch(() => props.visible, (val) => {
+  if (val) {
+    if (props.mode === 'edit' && props.announcement) {
+      formData.value = {
+        title: props.announcement.title,
+        content: props.announcement.content,
+        category: props.announcement.category as any,
+        priority: props.announcement.priority,
+        is_pinned: props.announcement.is_pinned,
+        is_important: props.announcement.is_important,
+      }
+    } else {
+      formData.value = {
+        title: '',
+        content: '',
+        category: 'notice',
+        priority: 0,
+        is_pinned: false,
+        is_important: false,
+      }
+    }
+    formRef.value?.clearValidate()
+  }
+})
+
+function handleClose() {
+  emit('update:visible', false)
+}
+
+async function handleSave() {
+  if (!formRef.value) return
+
+  try {
+    await formRef.value.validate()
+    emit('save', formData.value)
+  } catch (error) {
+    console.error('表单验证失败', error)
+  }
+}
+</script>
+
+<template>
+  <el-dialog
+    :model-value="visible"
+    :title="dialogTitle"
+    width="800px"
+    @close="handleClose"
+  >
+    <el-form
+      ref="formRef"
+      :model="formData"
+      :rules="rules"
+      label-width="100px"
+    >
+      <el-form-item label="公告标题" prop="title">
+        <el-input
+          v-model="formData.title"
+          placeholder="请输入公告标题"
+          maxlength="255"
+          show-word-limit
+        />
+      </el-form-item>
+
+      <el-form-item label="公告分类" prop="category">
+        <el-select v-model="formData.category" placeholder="请选择分类">
+          <el-option
+            v-for="cat in categories"
+            :key="cat.value"
+            :label="cat.label"
+            :value="cat.value"
+          >
+            <el-icon style="margin-right: 8px"><component :is="cat.icon" /></el-icon>
+            {{ cat.label }}
+          </el-option>
+        </el-select>
+      </el-form-item>
+
+      <el-form-item label="公告内容" prop="content">
+        <el-input
+          v-model="formData.content"
+          type="textarea"
+          :rows="12"
+          placeholder="支持 Markdown 或 HTML 格式，可以使用标题、列表、链接等"
+        />
+      </el-form-item>
+
+      <el-form-item label="优先级">
+        <el-input-number
+          v-model="formData.priority"
+          :min="0"
+          :max="999"
+          placeholder="数值越大越靠前"
+        />
+        <span style="margin-left: 12px; color: #909399; font-size: 13px">数值越大越靠前，默认为0</span>
+      </el-form-item>
+
+      <el-form-item label="标记">
+        <el-checkbox v-model="formData.is_pinned">置顶</el-checkbox>
+        <el-checkbox v-model="formData.is_important" style="margin-left: 20px">重要</el-checkbox>
+      </el-form-item>
+    </el-form>
+
+    <template #footer>
+      <el-button @click="handleClose">取消</el-button>
+      <el-button type="primary" @click="handleSave">保存</el-button>
+    </template>
+  </el-dialog>
+</template>
+
+<style scoped>
+:deep(.el-textarea__inner) {
+  font-family: 'SF Mono', 'Menlo', 'Consolas', monospace;
+  font-size: 13px;
+  line-height: 1.6;
+}
+</style>