feat: show group announcement read count and send read receipts

Backend added read_count/member_count/read_summary to the announcement
payload in today's deploy (previously confirmed absent). Wire it up:
- groups.ts: add the three fields to the GroupAnnouncement type.
- GroupAnnouncementModal.tsx: render "已读N/M" per README §4.8, and
  call readGroupAnnouncement() on open -- that endpoint existed in
  groups.ts already but was never called from anywhere, so opening an
  announcement never told the server this member had read it.

web/src/features/groups/GroupAnnouncementModal.tsx

@@ -1,10 +1,10 @@
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 
 import { Icon } from '@/app/Icon';
 import { Toast } from '@/app/Toast';
-import { getAllGroupMembers, getLatestGroupAnnouncement, updateCurrentGroupAnnouncement, type GroupRole } from '@/lib/api/groups';
+import { getAllGroupMembers, getLatestGroupAnnouncement, readGroupAnnouncement, updateCurrentGroupAnnouncement, type GroupRole } from '@/lib/api/groups';
 import { avatarBgClass, avatarLetter } from '@/lib/avatar';
 import { formatEventTimestamp } from '@/lib/messages/time';
 import { useEscapeKey } from '@/lib/hooks';
@@ -24,12 +24,15 @@ interface GroupAnnouncementModalProps {
  * corrections from real responses (2026-09-08, real group `grpvrdHqKqkCq0`): the publisher comes
  * back as a flat `author: "usr..."` id, not the `published_by` object this file originally guessed
  * -- resolved here against the group's member list, same as any other bare user id in this API.
- * And `read_count`/`member_count` are confirmed ABSENT (not just unconfirmed), so dropped rather
+ * When nothing has been published yet, the endpoint still returns `200` with every field
- * than kept as permanently-dead optional-chained code. When nothing has been published yet, the
+ * zero-valued, never `404`/`null` -- checked below via `content` emptiness, not object existence (a
- * endpoint still returns `200` with every field zero-valued, never `404`/`null` -- checked below
+ * real bug this file had before this fix: `!query.data` was never true, so it rendered a blank
- * via `content` emptiness, not object existence (a real bug this file had before this fix: `!query.
+ * "announcement" -- empty name, empty date -- instead of the empty state).
- * data` was never true, so it rendered a blank "announcement" -- empty name, empty date -- instead
+ * `read_count`/`member_count` were confirmed ABSENT on 2026-09-08; the 2026-09-18 backend deploy
- * of the empty state).
+ * added them (see groups.ts's own comment on `GroupAnnouncement`), so this now renders "已读N/M" as
+ * README §4.8 originally called for, and calls `readGroupAnnouncement` on open -- that REST endpoint
+ * existed in groups.ts since before this fix but was never actually called from anywhere, so opening
+ * an announcement never told the server "this member has read it" at all.
  */
 export function GroupAnnouncementModal({ topicId, canEdit, onClose }: GroupAnnouncementModalProps) {
   const queryClient = useQueryClient();
@@ -42,6 +45,22 @@ export function GroupAnnouncementModal({ topicId, canEdit, onClose }: GroupAnnou
   const membersQuery = useQuery({ queryKey: ['group-members', topicId], queryFn: () => getAllGroupMembers(topicId) });
   const author = membersQuery.data?.find((m) => m.user_id === query.data?.author);
 
+  // Fire-and-forget: viewing the announcement IS "reading" it, matching how the P2P/group message
+  // read receipt (native `{note what:"read"}`) already works elsewhere in this app -- no explicit
+  // "mark as read" button anywhere in the mock. Guarded on a real, non-empty `announcement_id` so
+  // this doesn't fire for the zero-valued "nothing published yet" response, and keyed on that id (not
+  // just "modal opened") so re-opening the SAME announcement doesn't re-send needlessly, though the
+  // endpoint's own idempotency key means it would be harmless either way.
+  const announcementId = query.data?.announcement_id;
+  useEffect(() => {
+    if (!announcementId) return;
+    // Invalidate (not just fire-and-forget) so THIS viewer's own read immediately bumps the
+    // displayed count instead of showing a stale pre-read number until some unrelated refetch.
+    readGroupAnnouncement(topicId, announcementId)
+      .then(() => queryClient.invalidateQueries({ queryKey: ['group-announcement', topicId] }))
+      .catch(() => {});
+  }, [topicId, announcementId, queryClient]);
+
   const saveMutation = useMutation({
     mutationFn: (content: string) => updateCurrentGroupAnnouncement(topicId, content, query.data?.version ?? 0),
     onSuccess: (data) => {
@@ -120,6 +139,19 @@ export function GroupAnnouncementModal({ topicId, canEdit, onClose }: GroupAnnou
                 <span className="text-xs text-text-tertiary-3">发布于 {formatEventTimestamp(new Date(query.data.published_at))}</span>
               </div>
               <p className="m-0 mt-3 whitespace-pre-wrap text-sm leading-[1.7] text-text-body">{query.data.content}</p>
+              {/* README §4.8. Both the flat fields and `read_summary` carry the same two numbers
+                  (confirmed reading the backend's own payload builder) -- prefer the flat ones,
+                  fall back to the nested object in case a future response only sends one shape. */}
+              {(() => {
+                const readCount = query.data.read_count ?? query.data.read_summary?.read_count;
+                const memberCount = query.data.member_count ?? query.data.read_summary?.member_count;
+                if (readCount == null || memberCount == null) return null;
+                return (
+                  <p className="m-0 mt-2 text-xs text-text-tertiary-3">
+                    已读 {readCount}/{memberCount}
+                  </p>
+                );
+              })()}
             </>
           )}
         </div>

web/src/lib/api/groups.ts

@@ -409,11 +409,15 @@ export function getGroupSummary(topic: string) {
 
 /** Confirmed live (2026-09-08, real group `grpvrdHqKqkCq0`): `author` is a flat `usr...` id, not
  *  the `published_by` object this file originally guessed -- resolve it against the group's member
- *  list for a display name/role, same as any other bare user id in this API. `read_count`/
+ *  list for a display name/role, same as any other bare user id in this API. When nothing has been
- *  `member_count` are confirmed ABSENT from the real response (not just unconfirmed), and when
+ *  published yet the endpoint still returns `200` with every field zero-valued (`announcement_id:"",
- *  nothing has been published yet the endpoint still returns `200` with every field zero-valued
+ *  author:"", content:"", published_at:"", version:0`), never `404`/`null` -- callers must check
- *  (`announcement_id:"", author:"", content:"", published_at:"", version:0`), never `404`/`null` --
+ *  `content` for emptiness, not object existence.
- *  callers must check `content` for emptiness, not object existence. */
+ *  `read_count`/`member_count`/`read_summary` were confirmed ABSENT on 2026-09-08 (dropped from this
+ *  type entirely, not just left optional) -- backend added them in the 2026-09-18 deploy (confirmed
+ *  by reading `groupAnnouncementPayload` in `im_be/server/im_http.go`, which now sets all three).
+ *  Optional here because an old cached response (or a tenant not yet on the new deploy) may still
+ *  omit them. */
 export interface GroupAnnouncement {
   announcement_id: string;
   topic: string;
@@ -422,6 +426,9 @@ export interface GroupAnnouncement {
   published_at: string;
   author: string;
   permissions?: { can_edit?: boolean };
+  read_count?: number;
+  member_count?: number;
+  read_summary?: { read_count: number; member_count: number };
 }
 
 export function getLatestGroupAnnouncement(topic: string) {