web: draw the chrome from sr-ht-ecore

The nav/service-switcher, the brand, the login block and the environment
banner were a copy of code every custom service on this instance carries.
The copies drifted, so they now come from one place:
sourcecraft.dev/bigbes/sr-ht-ecore/chrome.

web/chrome.go is gone. New builds one chrome.Service from the shared
config.ini and hands it the discovered stylesheet; view() asks it for a
chrome.Page per request, which viewData embeds rather than copies, so a
field ecore adds later arrives here without an edit. The layout invokes
"srht-env-banner" and "srht-nav" instead of the markup it used to spell
out, the landing's repository listing goes through "srht-repo-list", and
the FuncMap starts from chrome.Funcs() with the local shortsha deleted.

Two ecore policies differ from what this service did and are adopted as
the instance's: the brand label is the config section's short name rather
than a literal "compare", and the origins are trimmed of a trailing
slash. What stays local is what is compare's alone: the bundle href, the
diff-status colours, and the container-fluid the two diff views ask for.

Tests move to testify; the nav ordering/exclusion test goes with the code
it tested, and a test that the layout invokes the chrome at all replaces
it.

web: git.sr.ht-style dashboard and unified nav brand

Adopt the family look the dolt service already has. The nav brand
becomes circle icon + site name + red service label (dropping the
hub-origin variant), matching every other service on the instance. The
logged-in index turns into the two-column dashboard: a sidebar with the
service blurb and the jump form, and the repositories as shared-theme
event-list cards — name, visibility as small muted text (non-public
only, lowercase), description underneath. The big in-page h2 lives on
only for anonymous visitors, where the nav has no user context yet.

web: http server, sourcehut chrome, compare/commit pages

Implement the compare.sr.ht HTTP layer over the committed core/gitx/authz
packages:

- Server assembly (New) from shared SourceHut config: [git.sr.ht] repos,
  [meta.sr.ht]/[compare.sr.ht] origins, [sr.ht] site-name/environment, and
  the hashed stylesheet resolved by globbing the embedded static FS.
- Routes: landing, repo page, compare (base..base...head, .patch escape
  hatch), single-commit (+.patch), embedded static assets, healthz. Compare
  form GET canonicalizes to a clean URL via 302.
- SourceHut chrome port to html/template: nav service switcher (canonical
  order, paste/pages/hub excluded, compare active), login/logout with
  return_to, environment banner.
- Embedded compare-data JSON (template.JS so html/template's script-context
  escaper leaves it verbatim; json.Marshal's HTML escaping blocks a
  </script> breakout) plus the vendored bundle.js scaffold.
- httptest coverage with a stub Authorizer and a real git-CLI fixture repo.

The cmd layer must wire, in order: RealIP, Recoverer, Logger,
config.Middleware(conf,"compare.sr.ht"), authz.Middleware.