fix(web): code review round 2 -- messaging UI bugs and design fidelity

- DetailColumn: remark-edit draft and the history-search modal's open
  state weren't reset on contact switch (no key={topicId}, same long-
  lived instance) -- could silently save a typed remark against the
  WRONG contact. History-search state lifted to MessagesLayout so
  ConversationView's header search icon (previously wired to nothing)
  can open the same modal
- ConversationView: loadOlder's in-flight "restore scroll position"
  callback and its loading flag weren't guarded against a conversation
  switch happening mid-request, and the scroll-to-bottom effect keyed
  only on messages.length so two conversations that happened to load
  the same page size never got scrolled down on open
- ChatListColumn: pinned 系统消息/文件助手 rows used solid-color-square
  avatars like regular contacts; the actual design-code source
  (ChatList.dc.html) and reference screenshots use a light tinted
  background with a colored icon instead
- MessageBubble: own ("我") messages hash-colored their avatar like any
  contact instead of the design's fixed accent blue
- Every send/recall/favorite/forward action swallowed its own failure
  silently (empty .catch or, for forward, no catch at all) -- moved
  Toast from features/auth into app/ (shared) and wired real error
  feedback through it
- MessageContextMenu: added the recall countdown label, Escape-to-close,
  and viewport-edge clamping (a right-click near the window edge could
  render partially off-screen)
- EmojiPanel now defaults to 最近 per the reference (was defaulting to
  表情)
- HistorySearchModal: pagination (searchTopicMessages/getTopicMedia both
  supported cursor/next_cursor already, nothing used it -- silently
  truncated at one page), Escape-to-close, and a brief "没有找到" flash
  on the first keystroke before the debounce caught up
- A handful of design-token corrections (--shadow-btn, NavRail's border/
  spacing, LoginPage's logo radius/brand-panel padding, a placeholder
  icon color) verified against screens/*.dc.html's exact pixel values

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

web/src/app/AppShell.tsx

@@ -15,8 +15,11 @@ import { NavRail } from './NavRail';
 export function AppShell() {
   const contacts = useChatList();
   const bootstrap = useBootstrap();
+  // "sys"/"slf" have their own authoritative unread source in entry_badges (bootstrap.ts) -- excluded
+  // here in case they ever also show up as regular Tinode subscriptions in `contacts`, which would
+  // otherwise double-count the same unread messages into this badge.
   const unreadMessages =
-    contacts.reduce((sum, c) => sum + (c.unread || 0), 0) +
+    contacts.reduce((sum, c) => (c.topic === 'sys' || c.topic === 'slf' ? sum : sum + (c.unread || 0)), 0) +
     (bootstrap.data?.entry_badges?.system_unread_count || 0) +
     (bootstrap.data?.entry_badges?.file_assistant_unread_count || 0);