为项目页增加返回主站入口

在项目页头部增加返回主站按钮。

将返回动作接入平台首页路由。

补充项目页返回主站测试与文档说明。

src/components/rpg-entry/RpgEntryFlowShell.agent.interaction.test.tsx

@@ -9251,6 +9251,22 @@ test('desktop project tab switch clears creation primary styling', async () => {
   );
 });
 
+test('project page header can return to main site', async () => {
+  const user = userEvent.setup();
+  mockDesktopPlatformLayout();
+  window.history.replaceState(null, '', '/project');
+  render(<TestWrapper withAuth />);
+
+  expect(await screen.findByRole('main', { name: '项目' })).toBeTruthy();
+
+  await user.click(screen.getByRole('button', { name: '返回主站' }));
+
+  await waitFor(() => {
+    expect(screen.queryByRole('main', { name: '项目' })).toBeNull();
+  });
+  expect(window.location.pathname).toBe('/');
+});
+
 test('direct mobile creation home keeps landing content without mobile nav entry', async () => {
   window.history.replaceState(null, '', '/creation');
   const { container } = render(<TestWrapper withAuth />);