fix: add HTTP Range support to /filedownload

Rocket's FileServer returns 200 + full body for every request, so
browsers disable the resume button on interrupted multi-GB downloads.
Replace the FileServer mount with a custom range-aware handler that
advertises Accept-Ranges, parses single-range Range headers (bytes=N-M,
bytes=N-, bytes=-N), and returns 206 + Content-Range for the requested
slice. Invalid ranges return 416 with bytes */N.

Closes #153

Author: dobby-coder[bot]