web: draw the chrome from sr-ht-ecore

The nav/service-switcher, the login block, the environment banner and the
brand were this service's own copy of code compare.sr.ht had already copied
from somewhere else. They come from sourcecraft.dev/bigbes/sr-ht-ecore/chrome
now: one chrome.Service built at startup from the shared config.ini, one
chrome.Page per request, embedded in viewData so the shared partials find
their fields on the dot.

web/chrome.go is gone — buildNav, navItem, canonIndex, the login/logout/
profile URL building and the chrome half of viewData with it. The layout
renders srht-env-banner and srht-nav instead of the local markup, the landing
page's space list renders through srht-repo-list, and the template FuncMap
starts from chrome.Funcs() (the local shortsha was a duplicate of the shared
one). sameOrigin and the login redirect ask the chrome for our origin rather
than keeping a second copy that could disagree with the links on the page.

Three of ecore's policies differ from what this service did, and win, per that
package's own doc: [sr.ht]site-name defaults to "sr.ht" rather than
"sourcehut" and [sr.ht]environment to "development" rather than "production"
when the key is absent, and the brand carries a fixed 15rem min-width so the
switcher starts at the same x on every service. The instance's config.ini sets
both keys, so on it only the brand width is visible.

The nav tests that only restated ecore's rules — switcher order, the
paste/pages/hub exclusion, the shape of a login URL — are dropped; ecore tests
those. What is left covers this service's seam: that the identity authn
resolved is the one the chrome is handed.