chrome: give the brand both halves, and share the login URL and time helpers

The brand was upstream's minus its hub link: core.sr.ht points the whole
brand at hub when the instance has one and drops the red service label,
while this package kept the label and pointed at the service root. Hub is
excluded from the switcher, so that left the chrome with no route to hub at
all. Split the brand in two — the site name to hub (to the service root on
an instance without one), the label to the service root — and neither half
has to lose. The label keeps its own <span class="text-danger"> because the
theme colours ".navbar-light .navbar-brand a", which outranks .text-danger
and would repaint a red <a> white in dark mode.

LoginURLFor exports the line Page already computed. A handler that gates a
page behind login was building an entire Page to read one field off it.

RelTime/AbsTime come up from the services, where the copies had already
drifted over what to print for a future instant: "in 3 hours" on one
service, "just now" on the next, for the same timestamp.

bearer: the shared working-token validator of SPEC ch. 6

grants: the shared grant vocabulary of tokens.sr.ht

chrome: shared page chrome for the instance's custom services

Extract the nav/chrome idiom that compare, spec, dolt and cover each
carried as a private copy (and bench was about to fork as a sixth):
BuildNav over the shared config.ini with the canonical ordering and
paste/pages/hub exclusions, a per-request Page with login/logout/profile
URLs against meta's unified login, embedded srht-nav / srht-env-banner
partials (circle brand + red service label + switcher + login box), and
the generic dict/shortsha template helpers.

Policy decisions the copies had drifted on are baked in: switcher only
for authenticated viewers, hub excluded in the builder rather than the
template, profile link prefers hub's ~username page, environment name
uppercased for the banner. Service-specific needs surfaced by the cover
survey get seams instead of copies: ExtraNav for extra switcher entries,
ContainerClass for full-bleed pages, the config section as an explicit
parameter for the active check.