feat(AI设计): 支持输入框回车发送

需求：AI 设计输入框监听 Enter 并发送消息。

实现：复用现有发送链路，保留 Shift+Enter 换行并保护输入法组合状态；补充聚焦回归测试。

tests/unit/image-canvas-page.test.tsx

@@ -372,6 +372,42 @@ describe('ImageCanvas Workspace-first design experience', () => {
     expect(confirmImageWorkspaceGenerationMock).not.toHaveBeenCalled();
   });
 
+  it('sends the design message with Enter', async () => {
+    render(<MemoryRouter><ImageCanvas /></MemoryRouter>);
+    await screen.findByText('云端角色设定');
+    const composer = screen.getByLabelText('设计需求');
+
+    fireEvent.change(composer, {
+      target: { value: '让画面增加一层晨雾' },
+    });
+    fireEvent.keyDown(composer, { key: 'Enter' });
+
+    await waitFor(() => expect(sendImageWorkspaceMessageMock).toHaveBeenCalledWith(
+      'workspace-cloud',
+      1,
+      '让画面增加一层晨雾',
+      expect.stringMatching(/^turn-/),
+    ));
+    expect(composer).toHaveValue('');
+    expect(screen.getByTestId('image-workspace-composer'))
+      .toHaveTextContent('Enter 发送 · Shift+Enter 换行');
+  });
+
+  it('keeps Shift+Enter for newlines and ignores Enter during IME composition', async () => {
+    render(<MemoryRouter><ImageCanvas /></MemoryRouter>);
+    await screen.findByText('云端角色设定');
+    const composer = screen.getByLabelText('设计需求');
+
+    fireEvent.change(composer, {
+      target: { value: '正在输入中文设计描述' },
+    });
+    fireEvent.keyDown(composer, { key: 'Enter', shiftKey: true });
+    fireEvent.keyDown(composer, { key: 'Enter', isComposing: true });
+
+    expect(sendImageWorkspaceMessageMock).not.toHaveBeenCalled();
+    expect(composer).toHaveValue('正在输入中文设计描述');
+  });
+
   it('renders the pending user turn and assistant deltas while the command is running', async () => {
     const response = deferred<DesignWorkspace>();
     sendImageWorkspaceMessageMock.mockReturnValueOnce(response.promise);