boards: a card can come back once it is done

`:repeat weekly` gives a card a rule: daily, weekly, monthly, yearly, or
every +Nd, +Nw, +Nm, +Ny. Finishing it, by a move into the last list or by
the API's done, leaves a copy at the bottom of the inbox with the same
title, notes, document and rule, due on the next date. The finished card
gives its rule up in the same transaction, so reopening it and finishing it
again makes no second copy.

The next date is counted on the calendar in WORKSPACE_TIMEZONE, so noon
stays noon across a change of the clocks, and months are counted from the
old due date rather than step by step, so a card due on the 31st comes back
on the 31st wherever a month has one. A card finished late skips the dates
already past; one without a date counts from the moment it was finished.

A move and an edit answer with `again`, the copy, so the status line can
say when the card comes back. The board and the agenda show the rule as
`↻ weekly`, the rail has a button for it, and `:` in the agenda now opens
the line with the agenda kept up, so `:repeat` knows which card.

The rule travels in `.workspace.json` without a format bump: an older build
ignores the field and imports the card as one that does not repeat. An
import drops a rule it cannot read.

web/src/lib/Help.svelte

@@ -79,6 +79,7 @@
     ['Enter', 'to the card, on its own board — or to the entry'],
     ['Space', 'done, where it stands'],
     ['D', 'due date: fri, tm, +3d … — :due on the line'],
+    [':', 'the line, with the agenda kept up — :repeat weekly acts on the card'],
     ['H / L', 'to the list left / right on its board'],
     ['T', "today's diary entry"],
     ['t · q · Esc', 'close the agenda'],