fix(chat): align input area elements vertically centered

- Change flex container from items-end to items-center
- Use top-1/2 -translate-y-1/2 for send button vertical centering
- Adjust paperclip button padding for consistent sizing

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

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

@@ -280,13 +280,13 @@ export function InputArea({
       )}
 
       {/* 输入区域 */}
-      <div className="flex items-end gap-3">
+      <div className="flex items-center gap-3">
         {/* 文件上传按钮 */}
         <button
           onClick={openFilePicker}
           disabled={disabled || isUploading}
           className={clsx(
-            'p-3 rounded-xl transition-all',
+            'p-2.5 rounded-xl transition-all flex-shrink-0',
             disabled || isUploading
               ? 'bg-secondary-100 text-secondary-400 cursor-not-allowed'
               : 'bg-secondary-100 text-secondary-600 hover:bg-secondary-200',
@@ -333,7 +333,7 @@ export function InputArea({
             onClick={handleSubmit}
             disabled={disabled || isUploading || (!message.trim() && pendingFiles.length === 0)}
             className={clsx(
-              'absolute right-2 bottom-2 p-2 rounded-lg transition-all',
+              'absolute right-2 top-1/2 -translate-y-1/2 p-2 rounded-lg transition-all',
               (message.trim() || pendingFiles.length > 0) && !disabled && !isUploading
                 ? 'bg-primary-600 text-white hover:bg-primary-700'
                 : 'bg-secondary-100 text-secondary-400 cursor-not-allowed',