Fix acquisition bugs: camera-agnostic templates, mkdir, letterbox

- observation.py + fillheader.py: resolve camera device from
  params["camera"] instead of hardcoded cam; use viewer_fits_path
  per device so scicam/teccam FITS land in their viewer.ini paths
- stx.py + atik.py: mkdir -p before writing download file
- fits-viewer.js: letterbox image in square canvas instead of
  stretching; _canvasToImage accounts for letterbox offset

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>