fix(qrcode): 固定二维码预览图片尺寸，避免布局抖动

- 设置 QR_PREVIEW_IMAGE 固定尺寸 250x250
- 与 QRious 生成的二维码大小保持一致
- 解决输入内容变化时预览窗口大小跳动问题

config/pageTheme.ts

@@ -685,8 +685,8 @@ export const qrCodePageStyles = {
     width: '100%',
   } as const,
   QR_PREVIEW_IMAGE: {
-    maxWidth: '100%',
+    width: 250,
-    height: 'auto',
+    height: 250,
     display: 'block',
   } as const,
   QR_PREVIEW_ACTIONS: {