feat: add announcement action hooks with TDD

Add useAnnouncementActions hook for marking announcements as read and useAnnouncementUnreadCount hook for fetching unread announcement count. Both hooks follow TDD principles with complete test coverage.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hooks/use-announcement-unread-count.ts

@@ -0,0 +1,30 @@
+import { root } from '@repo/core'
+import { AnnouncementController, type UnreadAnnouncementCountResult } from '@repo/sdk'
+import { useState } from 'react'
+
+export const useAnnouncementUnreadCount = () => {
+  const [loading, setLoading] = useState(false)
+  const [error, setError] = useState<Error | null>(null)
+  const [data, setData] = useState<UnreadAnnouncementCountResult>()
+
+  const refetch = async () => {
+    setLoading(true)
+    setError(null)
+    try {
+      const announcementController = root.get(AnnouncementController)
+      const result = await announcementController.getUnreadCount()
+      setData(result)
+    } catch (err) {
+      setError(err as Error)
+    } finally {
+      setLoading(false)
+    }
+  }
+
+  return {
+    data,
+    loading,
+    error,
+    refetch,
+  }
+}