fix(chat): hide textarea scrollbar when content fits

- Set overflow-hidden by default on textarea
- Only show scrollbar when content exceeds max height (200px)
- Fix scrollbar appearing on empty input

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/web-client/src/features/chat/presentation/components/InputArea.tsx

@@ -58,8 +58,13 @@ export function InputArea({
   useEffect(() => {
     const textarea = textareaRef.current;
     if (textarea) {
+      // 重置高度以获取正确的scrollHeight
       textarea.style.height = 'auto';
-      textarea.style.height = `${Math.min(textarea.scrollHeight, 200)}px`;
+      // 设置新高度，最小44px（单行），最大200px
+      const newHeight = Math.max(44, Math.min(textarea.scrollHeight, 200));
+      textarea.style.height = `${newHeight}px`;
+      // 只有当内容超过最大高度时才显示滚动条
+      textarea.style.overflowY = textarea.scrollHeight > 200 ? 'auto' : 'hidden';
     }
   }, [message]);
 
@@ -317,7 +322,7 @@ export function InputArea({
             disabled={disabled || isUploading}
             rows={1}
             className={clsx(
-              'w-full resize-none rounded-xl border border-secondary-200 bg-white px-4 py-3 pr-12',
+              'w-full resize-none overflow-hidden rounded-xl border border-secondary-200 bg-white px-4 py-3 pr-12',
               'text-secondary-900 placeholder:text-secondary-400',
               'focus:outline-none focus:ring-2 focus:ring-primary-500 focus:border-transparent',
               'disabled:bg-secondary-50 disabled:cursor-not-allowed',