keys: the command line remembers and completes

`↑` / `↓` (and `Ctrl-p` / `Ctrl-n`) walk the lines run before, newest
first, narrowed by what is already typed: `:se` then `↑` finds the last
`:set …`. Commands and `/` searches keep separate histories in
localStorage, a hundred lines each. Passwords never get here: they are
typed into the vault prompt, not the line.

Tab moves to the next match and Shift-Tab to the one before, with the match
marked in the list. Once a name is complete, Tab completes the argument:
the folders listed here plus `..` and `/` for `:cd` and `:mv`, space names
for `:space` and `:rmspace`, the options of `:set`, `:due`'s usual dates and
`:backup`'s subcommands. `:mv /` now moves to the root of the space, as
`:cd /` goes there.

While a search is live, `Ctrl-n` / `Ctrl-p` (and Tab) move the list cursor
through the results without leaving the line, and Enter opens the one lit
rather than only closing the line. Ctrl-P's search does the same.

After a command the focus goes back where it was, or to the editor while
writing, instead of being left on <body> for the window handler to catch.

web/src/lib/Help.svelte

@@ -28,16 +28,16 @@
     ['Ctrl / Shift-click', 'pick a row up · every row up to it — drag them onto a folder'],
     ['t', 'agenda — late, due this week, in hand, on every board'],
     ['T', "today's diary entry, started if there is none"],
-    ['Ctrl-P', 'find anything, in every space'],
+    ['Ctrl-P', 'find anything, in every space — Enter goes to the result lit'],
     ['b', "open this space's board"],
     ['c', "capture a card into this space's inbox, without leaving here"],
     ['Esc · q', 'close the document'],
-    ['/', 'full-text search, inside the current space'],
+    ['/', 'full-text search, inside the current space — Tab or Ctrl-N/P walk the results, Enter opens one'],
     ['n', 'clear the search'],
     ['s', 'toggle sharing for the document — asks before taking it back'],
     ['y', 'mint a new share link and copy it — asks, the old one stops working'],
     ['x · dd', 'delete what has focus — the row in the list, the document in its pane (asks y/N)'],
-    [':', 'command line'],
+    [':', 'command line — Tab / Shift-Tab cycle the matches, then the argument; ↑ ↓ recall what ran before'],
     ['Ctrl-S', 'save, from anywhere'],
     ['?', 'this screen — in the editor too'],
   ]