feat: Implement background depth distance feature for depth processing

- Added `background_depth_distance` property to `ActiveJob` and `RunJob` classes to manage background depth settings.
- Updated job execution to handle background depth distance, allowing for fixed negative displacement in depth calculations.
- Introduced new functions in the worker module to apply background depth distance to depth frames and manage original background depth.
- Enhanced tests to cover new functionality, ensuring correct behavior of background depth processing and reference depth extraction.
- Refactored job submission to run in a separate thread, improving responsiveness during execution.

README.md

@@ -30,7 +30,7 @@ DA3、BEN2 源码包和 uv 已内置在扩展里，安装时直接从本地解
 2. 右键选择 `Convert to Depth Plane`
 3. 在对话框中选择：
    - `Depth from`：`Original` 直接用原图，`AI Generated` 让 AI 估算深度
-   - 需要去背景时勾选 `Remove Background`
+   - `Background Depth Distance` 为 `0` 时不剔除背景；大于 `0` 时把背景设为固定距离，AI 与 Original 均支持
    - 其余选项保持默认即可
 4. 点击确认，稍等片刻就得到带位移的立体面片