Relative dates ("5 minutes ago", "in 2 hours") were computed once via
dayjs().fromNow() and never recomputed, so a view left open kept showing
the value from the moment it was rendered.
Compute the relative string against the shared, ticking `now` from
useGlobalNow() instead. This makes every reactive caller — <TimeDisplay>,
direct formatDateSince() calls, and formatDisplayDate() when the user's
date display is set to relative — re-render on the existing 60s tick.
Absolute date formats don't read `now`, so they never needlessly
re-render.
useGlobalNow can now be initialised from a plain helper rather than only
from a component, so its route-update hook is guarded with
getCurrentInstance().
2cc7c0b6f0