feat: 完善 AI 分割与工作区标注闭环

功能增加:

- 将视频导入和生成帧拆成两个明确动作，项目库生成帧时选择 FPS，工作区不再自动触发拆帧。

- 为工作区新增调整多边形工具，支持选中 mask、拖动顶点、边中点插点、双击边界按位置插点，并保留多 polygon 子区域编辑。

- 打通 AI 页 SAM2/SAM3 结果到工作区的联动，生成 mask 后自动选中，可在右侧分类树换标签，并推送到工作区继续编辑。

- 增强 Dashboard WebSocket 连接状态与心跳，使用真实 onopen/onclose/onerror 状态驱动前端显示。

- 完善 SAM3 external worker 适配，支持 box prompt、semantic 请求级阈值和 video tracker 路径。

bugfix:

- 修复 SAM2 文本语义误走自动分割的问题，改为提示使用点提示或切换 SAM3。

- 修复 SAM2 多候选重叠显示的问题，点提示和 auto fallback 默认只采用最高分候选。

- 修复 SAM2 反向点看起来无效的问题，带负点时启用背景过滤，过滤为空时移除旧候选。

- 修复 SAM3 单个 2D mask 结果无法转 polygon、低阈值 semantic 返回被默认阈值吞掉的问题。

- 修复 AI 页 mask 未选中导致分类树无法修改 SAM2 结果标签的问题。

测试和文档:

- 补充 CanvasArea、AISegmentation、ProjectLibrary、VideoWorkspace、Dashboard、websocket 和 SAM engine/API 测试。

- 新增 backend/tests/test_sam2_engine.py，覆盖 SAM2 单候选请求和 auto fallback 行为。

- 更新 README、AGENTS 和 doc 需求/设计/接口/测试矩阵，按当前实现冻结功能状态。

src/components/CanvasArea.test.tsx

@@ -206,16 +206,58 @@ describe('CanvasArea', () => {
         { x: 300, y: 150, type: 'neg' },
       ],
       box: { x1: 120, y1: 80, x2: 260, y2: 200 },
+      options: { auto_filter_background: true, min_score: 0.05 },
     }));
     expect(useStore.getState().masks).toHaveLength(1);
     expect(useStore.getState().masks[0]).toEqual(expect.objectContaining({
       id: 'mask-box',
       segmentation: [[30, 30, 70, 30, 70, 70]],
       points: [[150, 100]],
-      metadata: expect.objectContaining({ promptPointCount: 2 }),
+      metadata: expect.objectContaining({ promptPointCount: 2, promptNegativePointCount: 1 }),
     }));
   });
 
+  it('removes the SAM2 candidate when a negative point filters it out', async () => {
+    apiMock.predictMask
+      .mockResolvedValueOnce({
+        masks: [
+          {
+            id: 'mask-box',
+            pathData: 'M 10 10 L 90 10 L 90 90 Z',
+            label: 'AI Mask',
+            color: '#06b6d4',
+            segmentation: [[10, 10, 90, 10, 90, 90]],
+            bbox: [10, 10, 80, 80],
+            area: 6400,
+          },
+        ],
+      })
+      .mockResolvedValueOnce({ masks: [] });
+
+    const { rerender } = render(<CanvasArea activeTool="box_select" frame={frame} />);
+    const stage = screen.getByTestId('konva-stage');
+    fireEvent.mouseDown(stage, { clientX: 120, clientY: 80 });
+    fireEvent.mouseMove(stage, { clientX: 260, clientY: 200 });
+    fireEvent.mouseUp(stage, { clientX: 260, clientY: 200 });
+
+    await waitFor(() => expect(useStore.getState().masks).toHaveLength(1));
+
+    rerender(<CanvasArea activeTool="point_neg" frame={frame} />);
+    fireEvent.click(stage, { clientX: 180, clientY: 120 });
+
+    await waitFor(() => expect(apiMock.predictMask).toHaveBeenNthCalledWith(2, {
+      imageId: 'frame-1',
+      imageWidth: 640,
+      imageHeight: 360,
+      model: 'sam2',
+      points: [{ x: 180, y: 120, type: 'neg' }],
+      box: { x1: 120, y1: 80, x2: 260, y2: 200 },
+      options: { auto_filter_background: true, min_score: 0.05 },
+    }));
+    await waitFor(() => expect(useStore.getState().masks).toHaveLength(0));
+    expect(await screen.findByText(/反向点已排除当前候选区域/)).toBeInTheDocument();
+  });
+
   it('renders only masks that belong to the current frame', () => {
     useStore.setState({
       masks: [
@@ -250,6 +292,28 @@ describe('CanvasArea', () => {
     await waitFor(() => expect(useStore.getState().selectedMaskIds).toEqual(['m1']));
   });
 
+  it('keeps a mask selected when opening the workspace polygon editor from AI results', () => {
+    useStore.setState({
+      selectedMaskIds: ['m1'],
+      masks: [
+        {
+          id: 'm1',
+          frameId: 'frame-1',
+          pathData: 'M 0 0 L 10 0 L 10 10 Z',
+          label: 'A',
+          color: '#fff',
+          segmentation: [[0, 0, 10, 0, 10, 10]],
+        },
+      ],
+    });
+
+    render(<CanvasArea activeTool="edit_polygon" frame={frame} />);
+
+    expect(useStore.getState().selectedMaskIds).toEqual(['m1']);
+    expect(screen.getAllByTestId('konva-circle')
+      .filter((element) => element.getAttribute('data-fill') === '#ffffff')).toHaveLength(3);
+  });
+
   it('renders imported GT seed points for editable point regions', () => {
     useStore.setState({
       masks: [
@@ -415,6 +479,34 @@ describe('CanvasArea', () => {
     }));
   });
 
+  it('selects a polygon with the edit tool and inserts a vertex by double-clicking an edge', () => {
+    useStore.setState({
+      masks: [
+        {
+          id: 'draft-1',
+          frameId: 'frame-1',
+          pathData: 'M 10 10 L 90 10 L 90 40 Z',
+          label: 'Draft',
+          color: '#06b6d4',
+          saveStatus: 'draft',
+          segmentation: [[10, 10, 90, 10, 90, 40]],
+          bbox: [10, 10, 80, 30],
+        },
+      ],
+    });
+
+    render(<CanvasArea activeTool="edit_polygon" frame={frame} />);
+    const path = screen.getByTestId('konva-path');
+    fireEvent.click(path);
+    fireEvent.doubleClick(path, { clientX: 50, clientY: 10 });
+
+    expect(useStore.getState().masks[0]).toEqual(expect.objectContaining({
+      segmentation: [[10, 10, 50, 10, 90, 10, 90, 40]],
+      pathData: 'M 10 10 L 50 10 L 90 10 L 90 40 Z',
+      saveStatus: 'draft',
+    }));
+  });
+
   it('edits the selected polygon in a multi-polygon mask', () => {
     useStore.setState({
       masks: [