fix(backend): prevent path traversal in SPA catch-all route (#126)

[AAtomical]

Summary

Fixes #126 — arbitrary file read via path traversal in the Docker/production SPA catch-all route GET /{full_path:path} in new_ui/backend/main.py.

The route joined the caller-supplied full_path directly onto FRONTEND_DIST and called exists()/is_file() on the un-resolved path. pathlib's / join does not strip .. segments, and percent-encoded separators (%2F) and dots (%2E%2E) survive Starlette's URL normalisation because the path parameter is decoded after routing. An unauthenticated client could read any file the process could access (SSH keys, secrets, /etc/passwd) outside the build directory.

Fix

Resolve the joined path and verify it stays within the resolved FRONTEND_DIST using is_relative_to before serving; otherwise fall back to index.html. is_relative_to requires Python 3.9+, matching setup.py's python_requires>=3.9.

Verification

Reproduced the original leak and confirmed the fix blocks it via a FastAPI TestClient against both old and new handlers:

Request	Before	After
GET /app.js (legit asset)	200, served	200, served
GET /..%2F..%2F..%2Fsecret_key	200, key leaked	200, index.html (no leak)