feat(bulletin): add group bulletin board (信息公示板)

- Add bulletin_posts and bulletin_reads PocketBase collections with migrations
- Add BulletinPost/BulletinRead types and priority levels (low/normal/high/urgent)
- Add bulletin API (CRUD, read tracking, realtime subscription)
- Add bulletin Pinia store with pinned/normal sections and expiration filtering
- Add BulletinPinned header component for pinned announcements
- Add BulletinBoard tab component with sectioned list layout
- Add BulletinPostCard with priority tags, unread dots, and hover actions
- Add CreateBulletinDialog with title, content, priority, pinned, expiresAt
- Integrate bulletin tab into GroupView with Bell icon and real-time updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/bulletin.ts

@@ -0,0 +1,170 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import type { BulletinPost, BulletinRead } from '@/types'
+import {
+  listBulletins,
+  listMyReads,
+  markBulletinAsRead,
+  createBulletin,
+  updateBulletin,
+  deleteBulletin,
+} from '@/api/bulletins'
+
+export const useBulletinStore = defineStore('bulletin', () => {
+  const posts = ref<BulletinPost[]>([])
+  const reads = ref<BulletinRead[]>([])
+  const loading = ref(false)
+
+  const now = computed(() => new Date())
+
+  const activePosts = computed(() => {
+    return posts.value.filter((p) => {
+      if (!p.expiresAt) return true
+      return new Date(p.expiresAt) > now.value
+    })
+  })
+
+  const pinnedPosts = computed(() =>
+    activePosts.value
+      .filter((p) => p.pinned)
+      .sort((a, b) => {
+        const prioOrder = { urgent: 0, high: 1, normal: 2, low: 3 }
+        return prioOrder[a.priority] - prioOrder[b.priority]
+      })
+  )
+
+  const normalPosts = computed(() =>
+    activePosts.value
+      .filter((p) => !p.pinned)
+      .sort((a, b) => {
+        const prioOrder = { urgent: 0, high: 1, normal: 2, low: 3 }
+        const prioDiff = prioOrder[a.priority] - prioOrder[b.priority]
+        if (prioDiff !== 0) return prioDiff
+        return new Date(b.created).getTime() - new Date(a.created).getTime()
+      })
+  )
+
+  const readPostIds = computed(() => new Set(reads.value.map((r) => r.post)))
+
+  function isRead(postId: string): boolean {
+    return readPostIds.value.has(postId)
+  }
+
+  function unreadCount(groupId: string): number {
+    return activePosts.value.filter(
+      (p) => p.group === groupId && !isRead(p.id)
+    ).length
+  }
+
+  async function loadPosts(groupId: string) {
+    try {
+      loading.value = true
+      const [fetchedPosts, fetchedReads] = await Promise.all([
+        listBulletins(groupId),
+        listMyReads(),
+      ])
+      posts.value = fetchedPosts
+      reads.value = fetchedReads
+    } catch (error) {
+      console.error('加载公告列表失败:', error)
+    } finally {
+      loading.value = false
+    }
+  }
+
+  async function markAsRead(postId: string) {
+    if (isRead(postId)) return
+    try {
+      const record = await markBulletinAsRead(postId)
+      if (record) {
+        reads.value.push(record)
+      }
+    } catch (error) {
+      console.error('标记已读失败:', error)
+    }
+  }
+
+  async function markAllAsRead(groupId: string) {
+    const unread = activePosts.value.filter(
+      (p) => p.group === groupId && !isRead(p.id)
+    )
+    await Promise.all(unread.map((p) => markAsRead(p.id)))
+  }
+
+  async function create(data: {
+    group: string
+    title: string
+    content: string
+    priority: 'low' | 'normal' | 'high' | 'urgent'
+    pinned: boolean
+    expiresAt?: string
+  }) {
+    try {
+      const post = await createBulletin(data)
+      posts.value.unshift(post)
+      return post
+    } catch (error: any) {
+      console.error('创建公告失败:', error)
+      throw error
+    }
+  }
+
+  async function update(
+    postId: string,
+    data: {
+      title?: string
+      content?: string
+      priority?: 'low' | 'normal' | 'high' | 'urgent'
+      pinned?: boolean
+      expiresAt?: string
+    }
+  ) {
+    try {
+      const post = await updateBulletin(postId, data)
+      const index = posts.value.findIndex((p) => p.id === postId)
+      if (index !== -1) {
+        posts.value[index] = { ...posts.value[index], ...post }
+      }
+      return post
+    } catch (error: any) {
+      console.error('更新公告失败:', error)
+      throw error
+    }
+  }
+
+  async function remove(postId: string) {
+    try {
+      await deleteBulletin(postId)
+      posts.value = posts.value.filter((p) => p.id !== postId)
+      reads.value = reads.value.filter((r) => r.post !== postId)
+    } catch (error: any) {
+      console.error('删除公告失败:', error)
+      throw error
+    }
+  }
+
+  function clear() {
+    posts.value = []
+    reads.value = []
+    loading.value = false
+  }
+
+  return {
+    posts,
+    reads,
+    loading,
+    activePosts,
+    pinnedPosts,
+    normalPosts,
+    readPostIds,
+    isRead,
+    unreadCount,
+    loadPosts,
+    markAsRead,
+    markAllAsRead,
+    create,
+    update,
+    remove,
+    clear,
+  }
+})