Add web UI for chunked video processing

[ssMBGit]

Summary

• Add a Flask-based web UI for uploading videos, preparing chunks, processing individual/all chunks, stitching output, and downloading results.
• Add VCR cleanup and Video2X/Real-ESRGAN treatment options alongside existing VRT treatments.
• Add resumable chunked browser uploads, job persistence, per-chunk status, deletion, elapsed/ETA display, and LAN-friendly serving.
• Add --save_dir to main_test_vrt.py and support sigma channels for single-video denoising inputs.

Verification

• Ran Python syntax checks for webui/app.py, main_test_vrt.py, and data/dataset_video_test.py.
• Exercised the UI locally on 0.0.0.0:17495 with large video upload, chunking, per-chunk processing, and process-all resume behavior.

Notes

• Runtime artifacts such as uploads, jobs, logs, model weights, and the local Video2X bundle are ignored and not included in this PR.