fix(canvas): block invalid video duration confirmation

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

@@ -544,6 +544,15 @@ describe('ImageCanvas Workspace-first design experience', () => {
     expect(durationSelect).toHaveValue('');
     expect(within(durationSelect).getAllByRole('option').map((option) => option.getAttribute('value')))
       .toEqual(['', '2', '6']);
+    expect(screen.getByRole('button', { name: '确认并开始生成' })).toBeDisabled();
+
+    fireEvent.change(screen.getByLabelText('设计需求'), {
+      target: { value: '确认生成' },
+    });
+    fireEvent.click(screen.getByRole('button', { name: '发送给设计 Agent' }));
+
+    expect(confirmImageWorkspaceGenerationMock).not.toHaveBeenCalled();
+    expect(await screen.findByRole('alert')).toHaveTextContent('请选择 2 至 6 秒的视频时长');
 
     fireEvent.change(durationSelect, { target: { value: '6' } });