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/api/messages.ts

@@ -68,3 +68,15 @@ export function getTopicMedia(topic: string, type: MediaType, cursor = '') {
   if (cursor) qs.set('cursor', cursor);
   return imGet<{ topic: string; items: MediaItem[]; next_cursor?: string }>(`/topics/${encodeURIComponent(topic)}/media?${qs.toString()}`);
 }
+
+/**
+ * A single message's "收藏" context-menu action is really just a one-item batch (docs/
+ * IM-前后端对接文档.md:1935): `POST /v1/im/favorites/batch {"messages":[{"topic","seq"}]}` ->
+ * `{"created_count","existing_count","failed":[]}`. `seq` travels as a string per the documented
+ * example, not a number.
+ */
+export function favoriteMessages(items: Array<{ topic: string; seq: number }>) {
+  return imPost<{ created_count: number; existing_count: number; failed: Array<{ seq: number; reason: string }> }>('/favorites/batch', {
+    messages: items.map((i) => ({ topic: i.topic, seq: String(i.seq) })),
+  });
+}