FEAT: Add viser-based remote viewer

[zoe1230]

This PR adds a browser-based remote viewer backend for Boxer, enabling interactive Prompt/Fusion/Tracker visualization over HTTP for SSH and headless workflows.

Changes

Viser backend integration:

• Added utils/viser_viewer.py with unified remote viewer implementations for prompt, fusion, and tracker modes
• Added shared utilities for OBB line generation, RGB loading, 3D-to-2D RGB overlay projection, trajectory rendering, camera pose lookup, and SDP loading
• Improved prompt-viewer responsiveness in web mode by forcing immediate websocket flush after detect/clear updates so new results appear without manual browser refresh

CLI integration for existing viewer entry points:

• Updated view_prompt.py to support remote backend selection and network binding options
• Updated view_fusion.py to support remote fusion controls, including seq-context loading, RGB panel toggles, SDP controls, and seek debounce
• Updated view_tracker.py to support remote tracker controls, including freeze mode, parameter apply delay, seek debounce, SDP toggle, and RGB 3D overlay toggle

Documentation updates:

• Updated README.md with optional viser installation and remote usage examples for tracker, fusion, and prompt viewers

How to Test

Note: The setup was tested on Linux with remote browser access.

1. Install optional remote viewer dependency
   uv pip install 'viser>=0.2.23'

2. Generate sample outputs first
   python run_boxer.py --input nym10_gen1 --max_n=90 --track --skip_viz

3. Tracker viewer over HTTP
   python view_tracker.py --input nym10_gen1 --viewer_backend viser --host 0.0.0.0 --port 8080

4. Fusion viewer over HTTP
   python view_fusion.py --input nym10_gen1 --viewer_backend viser --host 0.0.0.0 --port 8080 --viser_show_rgb

5. Prompt viewer over HTTP
   python view_prompt.py --input nym10_gen1 --viewer_backend viser --host 0.0.0.0 --port 8080

6. Open http://:8080 and verify:

   • timeline playback and frame seeking are stable
   • 3D OBBs render and update correctly
   • RGB panel and 3D overlays work when enabled
   • prompt detect/clear updates appear immediately without browser refresh
   • SDP visibility and point-size controls take effect

Notes

• Existing local viewer behavior is preserved
• Remote mode is opt-in through the viewer backend flag