refactor(announcements,messaging,notifications): V1+V2 审计重构 — i18n 命名空间独立 + 通知标题 i18n 化 + 服务端过滤 + 编排下沉 + 表单错误展示 + 架构图同步

V1 改进（已完成）:
- P0-4/P1-4/P1-5: 通知组件和 CRUD Action 从 messaging 迁移至 notifications 模块
- P1-5: 新增 getMessagesPageData / getAdminAnnouncementsPageData 编排函数
- P1-6: announcements schema 添加 superRefine 条件校验
- P1-7: 新增 useMessageSearch hook（防抖 + 请求竞态取消）+ 客户端分页 UI
- P1-9: deleteMessage 事务化
- P2-11: 全模块 trackEvent 埋点
- 全模块 i18n 接入 + Error Boundary + a11y 改进

V2 改进（本次完成）:
- V2-P0-1: 通知 i18n 命名空间独立（notifications.json），useTranslations 从 "messages" 切换到 "notifications"
- V2-P0-2: 公告/消息通知标题 i18n 化，Server Action 中使用 getTranslations 生成通知标题
- V2-P1-1: AnnouncementList 纯服务端过滤，移除客户端 useState/useMemo
- V2-P1-2: MessageList 客户端过滤仅在初始数据时执行，搜索结果由服务端按 tab 过滤
- V2-P1-3: 消息详情页编排下沉，新增 getMessageDetailPageData 编排函数
- V2-P1-4: 表单服务端校验错误展示（fieldErrors + aria-invalid）
- V2-P2-1: 轮询间隔常量化（POLL_INTERVAL_MS）
- V2-P2-2: 架构图同步（004 + 005）

src/modules/messaging/components/message-compose.tsx

@@ -23,6 +23,8 @@ import {
 import { sendMessageAction } from "../actions"
 import type { RecipientOption } from "../types"
 
+type FieldErrors = Record<string, string[]>
+
 export function MessageCompose({
   recipients,
   parentMessageId,
@@ -40,10 +42,16 @@ export function MessageCompose({
   const router = useRouter()
   const [isWorking, setIsWorking] = useState(false)
   const [receiverId, setReceiverId] = useState(defaultReceiverId ?? "")
+  const [fieldErrors, setFieldErrors] = useState<FieldErrors>({})
+
+  const getFieldError = (field: string): string | null => {
+    const errs = fieldErrors[field]
+    return errs && errs.length > 0 ? errs[0] : null
+  }
 
   const handleSubmit = async (formData: FormData) => {
     if (!receiverId) {
-      toast.error(t("messages.selectRecipient"))
+      toast.error(t("form.selectRecipient"))
       return
     }
     formData.set("receiverId", receiverId)
@@ -52,6 +60,7 @@ export function MessageCompose({
     }
 
     setIsWorking(true)
+    setFieldErrors({})
     try {
       const res = await sendMessageAction(null, formData)
       if (res.success) {
@@ -59,6 +68,10 @@ export function MessageCompose({
         router.push("/messages")
         router.refresh()
       } else {
+        // 展示字段级错误（来自 Zod 校验）
+        if (res.errors) {
+          setFieldErrors(res.errors)
+        }
         toast.error(res.message || t("messages.sendFailed"))
       }
     } catch {
@@ -85,7 +98,7 @@ export function MessageCompose({
           <div className="grid gap-2">
             <Label htmlFor="receiverId">{t("form.to")}</Label>
             <Select value={receiverId} onValueChange={setReceiverId} disabled={!!defaultReceiverId}>
-              <SelectTrigger>
+              <SelectTrigger aria-invalid={!!getFieldError("receiverId")}>
                 <SelectValue placeholder={t("form.toPlaceholder")} />
               </SelectTrigger>
               <SelectContent>
@@ -98,6 +111,9 @@ export function MessageCompose({
               </SelectContent>
             </Select>
             <input type="hidden" name="receiverId" value={receiverId} />
+            {getFieldError("receiverId") ? (
+              <p className="text-destructive text-xs">{getFieldError("receiverId")}</p>
+            ) : null}
           </div>
 
           <div className="grid gap-2">
@@ -108,7 +124,11 @@ export function MessageCompose({
               placeholder={t("form.subjectPlaceholder")}
               defaultValue={defaultSubject ?? ""}
               maxLength={255}
+              aria-invalid={!!getFieldError("subject")}
             />
+            {getFieldError("subject") ? (
+              <p className="text-destructive text-xs">{getFieldError("subject")}</p>
+            ) : null}
           </div>
 
           <div className="grid gap-2">
@@ -119,7 +139,11 @@ export function MessageCompose({
               placeholder={t("form.contentPlaceholder")}
               className="min-h-[200px]"
               required
+              aria-invalid={!!getFieldError("content")}
             />
+            {getFieldError("content") ? (
+              <p className="text-destructive text-xs">{getFieldError("content")}</p>
+            ) : null}
           </div>
 
           <CardFooter className="justify-end gap-2 px-0">