Add ChatGPT-style TotalSegmentator demo webapp ...

FastAPI + vanilla JS app under webapp/: upload a CT/MRI NIfTI volume and
ask in natural language (EN/JP) for a structure to segment. Reuses the
querys modality/target-parsing prompt idea from agents/c2f_tree_search.py
but drives real TotalSegmentator inference instead of just recommending a
dataset. Modality is auto-detected from voxel intensities, not the query
text. Results render as a tappable chat thumbnail that expands into an
interactive Niivue volume viewer.

Purely additive: does not touch agents/, utils/, dataset/, or the root
requirements.txt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>