91a031fdb3
A verbatim index.html booth (e.g. edict-design-brief: a rendered brief + its
.md) had no download affordance — the page is served raw with no gallery/per-file
chrome. Adds:
- GET /b/<name>/?download=1 -> streams the whole booth as <name>.zip (attachment)
- GET /b/<name>/<file>?dl=1 -> forces Content-Disposition: attachment (html/md/
text otherwise render inline with no easy save)
- a download link on the index card (the accessible spot for verbatim booths)
and the gallery header
Tests for both; verified live against edict-design-brief on nh3-dev :8090.