refactor: 规范化页面组件目录结构

- 将 QrCodePreview、ImageUploader 移动到 QrCode/components/
- 将 DecodeResultPaper、Base64ConverterSection、TextMode 移动到 Base64Converter/components/
- 提取 useTextMode Hook，遵循 UI + Hook 分离模式
- 简化 SwitchButtonGroup 组件，提取常量到外部

Co-authored-by: Cursor <cursoragent@cursor.com>

src/pages/QrCode/components/QrCodePreview.tsx

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+import React from 'react';
+import { Copy, Download } from 'lucide-react';
+import { useI18n } from '@/utils/chromeI18n';
+import { cn } from '@/lib/utils';
+import { Button } from '@/components/ui/button';
+
+// 继承原生 HTML Div 属性，方便外部无缝扩充类名或监听事件
+interface QrCodePreviewProps extends React.HTMLAttributes<HTMLDivElement> {
+  /** 二维码 Data URL */
+  qrCodeDataUrl: string;
+  /** 下载回调 */
+  onDownload: () => void;
+  /** 复制回调 */
+  onCopy: () => void;
+  /** 自定义占位文本，用于空状态提示 */
+  placeholderText?: string;
+}
+
+const QrCodePreview = ({
+  qrCodeDataUrl,
+  onDownload,
+  onCopy,
+  placeholderText,
+  className,
+  ...props
+}: QrCodePreviewProps) => {
+  const { t } = useI18n('qrCode');
+
+  // 空状态下的虚线骨架屏
+  if (!qrCodeDataUrl) {
+    return (
+      <div
+        className={cn(
+          'flex flex-col justify-center items-center min-h-[200px] border border-dashed border-input rounded-xl p-4 bg-muted/40',
+          className,
+        )}
+        {...props}
+      >
+        <p className="text-sm text-muted-foreground text-center">
+          {placeholderText || t('qrCode:qrCodeWillShow')}
+        </p>
+      </div>
+    );
+  }
+
+  return (
+    <div
+      className={cn(
+        'flex flex-col justify-center items-center border border-input rounded-xl p-4 bg-muted/40',
+        className,
+      )}
+      {...props}
+    >
+      <div className="flex flex-col items-center w-full max-w-xs">
+        {/*
+          二维码容器适配：
+          在暗黑模式下，纯黑白的二维码如果直接暴露在暗色背景下，会导致手机摄像头极难识别。
+          通过裹一层 bg-white 和 p-2，确保黑白对比度绝对安全，同时加入 shadow 增强卡片感。
+        */}
+        <div className="p-2 bg-white rounded-lg shadow-sm border border-border/40">
+          <img
+            src={qrCodeDataUrl}
+            alt="QR Code Preview"
+            className="w-48 h-48 max-w-full object-contain block animate-in fade-in duration-300"
+          />
+        </div>
+
+        <div className="flex w-full gap-2 mt-3">
+          <Button variant="outline" size="sm" onClick={onDownload} className="flex-1 h-8">
+            <Download className="w-3.5 h-3.5 text-muted-foreground" />
+            <span className="truncate text-xs">{t('qrCode:downloadButton')}</span>
+          </Button>
+
+          <Button variant="default" size="sm" onClick={onCopy} className="flex-1 h-8">
+            <Copy className="w-3.5 h-3.5" />
+            <span className="truncate text-xs">{t('qrCode:copyQrButton')}</span>
+          </Button>
+        </div>
+      </div>
+    </div>
+  );
+};
+
+export default QrCodePreview;