[Frontend] Add secure file-name handling for medical uploads

## Problem
User-controlled file names can create misleading UI, unsafe download names, or path-like values in logs and storage keys.
## Acceptance criteria
- Display names are escaped and normalized separately from storage identifiers.
- Path traversal, control characters, and excessive lengths are rejected or safely rewritten.
- Downloads use safe Content-Disposition values and preserve the original name only as metadata.
- Tests cover Unicode, bidi controls, traversal, duplicate names, and long names.

No files, tests, or entry points are named. Start by locating the frontend upload and download handling, then identify how display names, storage identifiers, logs, and Content-Disposition values are currently produced. Done means the stated rejection or rewriting rules are implemented and tests cover Unicode, bidi controls, traversal, duplicates, and long names.

Written by the indexing model from the issue text.