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__/index.test.tsx

@@ -12,7 +12,7 @@ vi.mock('@/config/features', async (importOriginal) => {
 });
 
 // Mock 子组件
-vi.mock('../TextMode', () => ({
+vi.mock('../components/TextMode', () => ({
   default: ({ onSwitchToImageMode }: { onSwitchToImageMode?: () => void }) => (
     <div data-testid="text-mode">
       TextMode
@@ -21,7 +21,7 @@ vi.mock('../TextMode', () => ({
   ),
 }));
 
-vi.mock('../Base64ConverterSection', () => ({
+vi.mock('../components/Base64ConverterSection', () => ({
   default: ({ mode }: { mode: string }) => <div data-testid={`${mode}-mode`}>{mode}</div>,
 }));