web: show how fresh a beads view is

handleView now reads the head commit of the rendered ref onto the envelope
(Head *browse.CommitInfo), and the beads and milestones headers carry a
shared beadsHead partial: <branch> · last commit <relative> · <short hash>,
the hash linking to the commit page and the exact stamp in the title.

The read is decoration on top of an answer: a database with no commits, or
a Log that fails, renders the page without the line rather than 500ing.

The relative time is a new ago func rather than chrome's reltime — it is
past-facing (clock skew reads as "just now", never "in 3 minutes") and
reads a package clock a test can pin.