feat(web): pagination, quote/reply, favorite, and single-message forward

- ConversationView: infinite-scroll "加载更多" -- scroll-to-40px triggers
  topic.msgHasMoreMessages()+getMessagesPage() to fill exactly the missing
  seq ranges (not naive since/before paging, since server-side deletions can
  punch holes in an otherwise-contiguous range), with scroll position
  preserved across the prepend.
- lib/tinode/send.ts: sendReply() ports the old app's exact quote
  construction (Drafty.replyContent truncates, Drafty.quote wraps with the
  sender's name, Drafty.append inlines the whole quote block ahead of the
  new text -- the receiving side never needs to look up the original
  message). forwardMessage() sends a single message directly to one picked
  target topic (ForwardPickerModal) -- simplified from the old app's
  stage-then-compose flow, trading that nicety for a much smaller dialog.
- lib/messages/drafty.tsx: added the 'QQ' Drafty style-tag case ported
  against the SDK's actual Drafty.quote() output shape (no `key`/`data` of
  its own -- verified by reading tinode.dev.js, not guessed).
- lib/api/messages.ts: favoriteMessages() -- a single message is just a
  one-item batch against POST /v1/im/favorites/batch (confirmed in
  docs/IM-前后端对接文档.md:1935).
- Context menu now shows 引用/复制/收藏/转发/多选/[撤回]/删除 in spec order
  (README §3.5); 翻译 still deliberately absent (no real backend for it,
  confirmed by research).

Verified end to end: sent a real quote reply and confirmed the exact wire
packet (head.reply, QQ fmt span, MN entity) and that it renders correctly
via the new QQ formatter case; favorite POSTed and returned created_count:1;
forward POSTed the expected {head:{forwarded:"topic:seq"}} packet.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/messages/actions.ts

@@ -1,22 +1,24 @@
 import type { StoredMessage } from 'tinode-sdk';
 
 import type { MessagePolicy } from '@/lib/api/bootstrap';
+import { isRecalled, isServerDeleted } from '@/lib/messages/dispatch';
 
 /** SDK-internal placeholder seq threshold for a not-yet-server-confirmed message (tinode.dev.js's
  *  own `LOCAL_SEQID`, not exported as a Tinode static -- verified against source, not guessed). */
 const LOCAL_SEQID = 0xfffffff;
 
-export type MessageAction = 'copy' | 'recall' | 'delete' | 'select';
+export type MessageAction = 'copy' | 'quote' | 'favorite' | 'forward' | 'recall' | 'delete' | 'select';
 
 /**
  * Which context-menu actions apply to one message. Scoped to P2P for now (no group-admin recall,
  * no self-topic hard-delete-always -- those need group role/topic-type knowledge Phase 3 will add).
- * 引用/收藏/翻译/转发 are deliberately not in this list yet: quote and forward need their own
- * composer-state flows, and there's no confirmed single-message favorite endpoint yet -- adding
- * them here before they actually work would just be dead menu items.
+ * 翻译 is deliberately never in this list -- confirmed via research that the old app has zero
+ * backend call or handler for it anywhere, only a decorative id in its mock; there's nothing real
+ * to wire up.
  */
 export function availableActions(msg: StoredMessage, myUserId: string | null, messagePolicy: MessagePolicy | undefined): MessageAction[] {
-  const actions: MessageAction[] = ['copy'];
+  if (isServerDeleted(msg) || isRecalled(msg)) return [];
+  const actions: MessageAction[] = ['quote', 'copy', 'favorite', 'forward'];
   const mine = !!myUserId && msg.from === myUserId;
   const isConfirmed = typeof msg.seq === 'number' && msg.seq < LOCAL_SEQID;
   const recallWindow = messagePolicy?.recall_window_seconds ?? 0;