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/Base64Converter/index.tsx

@@ -1,8 +1,8 @@
 import { useI18n } from '@/utils/chromeI18n';
 import { useStorageState } from '@/utils/useStorageState';
 import type { Base64ConverterPageMode } from '@/types/storage';
-import TextMode from './TextMode';
-import Base64ConverterSection from './Base64ConverterSection';
+import TextMode from './components/TextMode';
+import Base64ConverterSection from './components/Base64ConverterSection';
 import SwitchButtonGroup from '@/components/SwitchButtonGroup';
 
 const VALID_PAGE_MODES: readonly Base64ConverterPageMode[] = ['text', 'file', 'image'];