Project Editor, Editing, Spatial Subtitles, and Multi-Source Export

## Project Editor

- Added a dedicated **Project Editor** for batch projects.
- Automatically loads all valid video pairs into a project-level timeline.
- Trim, split, delete, and reorder clips from different video pairs in one workspace.
- Manage the combined project timeline and spatial subtitles together.
- Saved edits from individual video pairs can be used in the Project Editor.
- Use the completed project timeline when merging selected video pairs into one video.

## Editing and Timeline

- Added a dedicated **Editing & Subtitles** workspace for individual video pairs.
- Supports frame-accurate trimming, splitting, reordering, and ripple deletion.
- Supports undo and redo.
- Clips can be reordered by dragging them directly.
- Supports timeline zooming, fitting the entire sequence, and automatic edge scrolling.
- Preview and playback can begin from any selected frame.
- Added separate audio waveforms for the left-eye and right-eye videos.

## Spatial Subtitles

- Spatial subtitles are now integrated into the **Editing & Subtitles** workspace.
- Added box selection and multi-selection for subtitle clips.
- Multiple selected subtitles can be deleted together.
- Added **Apply** and **Apply to All** for editing either the current subtitle or every subtitle.
- Video editing and subtitle editing share the same undo and redo history.

## Multi-Source Merged Export

- Multiple video pairs can be merged into one continuous video.
- Clip ranges and playback order can be defined on the project timeline.
- Supports cross-source reordering, non-contiguous clips, hard cuts, and single-frame clips.
- Edited multi-source sequences can be exported using Full GPU processing.

## Audio Options

- Added a **Preserve Source Audio Quality** option.

## Preview and Source Settings

- Added horizon controls to fullscreen preview.
- Added **Reset Defaults** to Custom Source Settings, restoring the automatically detected parameters for the current source.