fix(core): use request host in exhibit URL display so dev shows dev.jandig.app (#822)

[SAY-5]

Fixes #822. Three exhibit-related templates hardcoded https://jandig.app/ for the URL display + share link, so on dev.jandig.app the user saw / copied a link pointing at production.

Files

• src/core/jinja2/core/exhibit_detail.jinja2, display {{ request.scheme }}://{{ request.get_host() }}/{slug}; anchor href switched to relative /{slug} (zero-cost, more idiomatic, no host concern)
• src/core/jinja2/core/exhibit_create_ar.jinja2 and exhibit_create_mr.jinja2, url-helper label uses the same dynamic host so the "Your exhibit URL will look like this" preview is honest on dev too

request is already provided to Jinja2 templates by Django (other templates in the repo use request.user / request.LANGUAGE_CODE).

home_v2.jinja2 still has https://jandig.app/docs/... links, those go to the docs site which is canonical-on-prod, intentionally not touched.

Test plan

• 3-file template change, 4 lines +/-
• No view/model code touched; production output unchanged because request.get_host() == 'jandig.app' there
• Dev render now shows dev.jandig.app/<slug>