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/features/messages/MessageContextMenu.tsx

@@ -11,7 +11,10 @@ interface MenuItem {
 }
 
 const ITEMS: Record<MessageAction, MenuItem> = {
+  quote: { action: 'quote', label: '引用', icon: 'format_quote' },
   copy: { action: 'copy', label: '复制', icon: 'content_copy' },
+  favorite: { action: 'favorite', label: '收藏', icon: 'inventory_2' },
+  forward: { action: 'forward', label: '转发', icon: 'reply' },
   recall: { action: 'recall', label: '撤回', icon: 'undo' },
   select: { action: 'select', label: '多选', icon: 'select_all' },
   delete: { action: 'delete', label: '删除', icon: 'delete_outline', danger: true },