拆分图片画布素材拖拽桥接

新增素材拖拽桥接 hook，承接素材拖向画布或文件夹的全局 pointer 监听

恢复认证弹窗 portal 渲染，避免全屏画布遮住账号入口

优化画布背景设置面板，补回当前色、色域、色相、预设、HEX 和恢复默认

补充素材拖拽、认证弹窗和背景面板回归测试并更新文档与 TRACKING

src/components/auth/PlatformAuthModalShell.test.tsx

@@ -8,7 +8,7 @@ import { PlatformAuthModalShell } from './PlatformAuthModalShell';
 test('renders auth modal shell with platform theme and auth card chrome', () => {
   const onClose = vi.fn();
 
-  render(
+  const { container } = render(
     <PlatformAuthModalShell
       title="账号入口"
       platformTheme="light"
@@ -22,6 +22,8 @@ test('renders auth modal shell with platform theme and auth card chrome', () =>
 
   const dialog = screen.getByRole('dialog', { name: '账号入口' });
 
+  expect(container.querySelector('[role="dialog"]')).toBeNull();
+  expect(document.body.contains(dialog)).toBe(true);
   expect(dialog.parentElement?.className).toContain('platform-theme--light');
   expect(dialog.className).toContain('platform-modal-shell');
   expect(dialog.className).toContain('platform-auth-card');