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/MessageBubble.tsx

@@ -14,7 +14,6 @@ interface MessageRowProps {
   mine: boolean;
   isGroup: boolean;
   peerName: string;
-  myUserId: string | null;
   selectMode?: boolean;
   selected?: boolean;
   /** README §3.5: "选中气泡加 outline:2px solid #1677ff" -- true while this message is the open
@@ -30,7 +29,6 @@ export function MessageRow({
   mine,
   isGroup,
   peerName,
-  myUserId,
   selectMode,
   selected,
   menuActive,
@@ -81,7 +79,9 @@ export function MessageRow({
         className={`flex flex-1 items-end gap-2.5 ${mine ? 'flex-row-reverse' : ''}`}
       >
         <span
-          className={`grid h-9 w-9 flex-none place-items-center rounded-lg text-sm font-medium text-white ${avatarBgClass(mine ? myUserId || '' : msg.from || peerName)}`}
+          className={`grid h-9 w-9 flex-none place-items-center rounded-lg text-sm font-medium text-white ${
+            mine ? 'bg-accent' : avatarBgClass(msg.from || peerName)
+          }`}
         >
           {avatarLetter(mine ? '我' : peerName)}
         </span>