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/features/messages/ChatListColumn.tsx

@@ -25,7 +25,7 @@ const PINNED_ENTRIES = [
     label: '系统消息',
     icon: 'notifications',
     route: '/chat/system',
-    colorClass: 'bg-warn',
+    colorClass: 'bg-warn-bg text-warn',
   },
   {
     key: 'file_assistant' as const,
@@ -33,7 +33,7 @@ const PINNED_ENTRIES = [
     label: '文件助手',
     icon: 'folder',
     route: '/files',
-    colorClass: 'bg-success',
+    colorClass: 'bg-success-bg text-success',
   },
 ];
 
@@ -75,7 +75,7 @@ export function ChatListColumn() {
               to={entry.route}
               className="grid min-h-[68px] grid-cols-[44px_minmax(0,1fr)_auto] items-center gap-3 border-b border-list-item bg-white px-4 py-2.5 no-underline hover:bg-[#f6f8fb]"
             >
-              <span className={`grid h-11 w-11 place-items-center rounded-lg text-white ${entry.colorClass}`}>
+              <span className={`grid h-11 w-11 place-items-center rounded-lg ${entry.colorClass}`}>
                 <Icon name={entry.icon} className="text-[22px]" />
               </span>
               <span className="flex min-w-0 flex-col gap-1">