fix(frontend): skip tracking effect update when caret is at end of text

Root cause: after onCategoryEnter sets value="@ai:", the MentionInput sync
effect sets innerHTML with the new content. The browser caret may be at
position 2 (not moved from the user's perspective), but prevCursorRef was
4. The tracking effect read DOM caret=2 and overwrote ms.setCursorOffset(4)
→ cursor jumped to position 2.

Fix: skip tracking effect update when:
- count (DOM caret) is at end of text (ms.value.length), AND
- prevCursorRef was also at end of text
This means the caret hasn't actually moved from the user's POV (just
positioned by the browser after innerHTML), so don't update state.

src/components/room/RoomChatPanel.tsx

@@ -104,9 +104,14 @@ const ChatInputArea = memo(function ChatInputArea({
       count += node.length;
     }
     if (count !== prevCursorRef.current) {
+      // Skip update when caret is at end of text (programmatic value change
+      // that already positioned the caret — the caret hasn't moved from the
+      // user's perspective). Only update on real user-initiated cursor movement.
+      if (count !== ms.value.length || prevCursorRef.current !== ms.value.length) {
         prevCursorRef.current = count;
         ms.setCursorOffset(count);
       }
+    }
   });
 
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