keys: n walks the search results, and a half-typed key shows

`n` / `N` move to the next / previous search result and open it beside the
results, so the next `n` goes on from there; a folder is only highlighted,
as going into it would end the search. Past the last result it starts
again at the first and says so, as vim's `wrapscan` does. Clearing the
search is left to `:noh` / `:clear`, as in vim, rather than to `n`.

After `g`, `d`, `]` or `[` the status line shows the key waiting for its
second half, like vim's `showcmd`, and forgets it after a second.

web/src/lib/Help.svelte

@@ -10,7 +10,7 @@
     ['f', 'fullscreen the focused pane; again or Esc to leave'],
     ['Alt-h / Alt-l', 'focus the list / the document, from the editor too'],
     ['j / k · ↓ ↑', 'move down / up the list — scroll the document when it has focus'],
-    ['gg / G', 'first / last row, or top / bottom of the document'],
+    ['gg / G', 'first / last row, or top / bottom of the document — a first g, d, ] or [ waits a second in the status line'],
     ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
     [']] / [[', 'next / previous heading of the document — in the editor too'],
     ['gO', 'the headings as a list: j/k, Enter jumps there'],
@@ -33,7 +33,7 @@
     ['c', "capture a card into this space's inbox, without leaving here"],
     ['Esc · q', 'close the document'],
     ['/', 'full-text search, inside the current space — Tab or Ctrl-N/P walk the results, Enter opens one'],
-    ['n', 'clear the search'],
+    ['n / N', 'next / previous search result, opened beside the results — :noh clears 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)'],