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/GeneratePanel.tsx

@@ -1,6 +1,6 @@
 import { Loader2, Pencil, QrCode } from 'lucide-react';
 import TextInputArea from '@/components/TextInputArea';
-import QrCodePreview from '@/components/QrCodePreview';
+import QrCodePreview from './QrCodePreview';
 import { useI18n } from '@/utils/chromeI18n';
 import { useQrCodeContext } from '../contexts/QrCodeContext';
 import { Button } from '@/components/ui/button';