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/__tests__/TextMode.test.tsx

@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { act, fireEvent, render, screen, waitFor } from '@testing-library/react';
-import TextMode from '../TextMode';
+import TextMode from '../components/TextMode';
 
 // Mock CopyButton
 vi.mock('@/components/CopyButton', () => ({