agenda: a card's date and list without going to its board

In the agenda, `D` asks for the highlighted card's due date on the command
line (`:due` now works there as well as on the board), and `H` / `L` send
the card to the list left or right of the one it is in, on its own board.
`L` into the last list is done, as on the board.

The command line now rises above an open overlay, and while it is up the
overlay's own keys stay out of its way, so typing a date does not walk the
agenda.

web/src/lib/Help.svelte

@@ -70,6 +70,8 @@
     ['j / k', 'down / up the cards'],
     ['Enter', 'to the card, on its own board'],
     ['Space', 'done, where it stands'],
+    ['D', 'due date: fri, tm, +3d … — :due on the line'],
+    ['H / L', 'to the list left / right on its board'],
     ['T', "today's diary entry"],
     ['t · q · Esc', 'close the agenda'],
   ]