trash: a delete can be undone

`x`, `dd`, `:delete` and `:rmspace` no longer end anything. What they
delete is moved, whole, under one hidden root, and `u` puts the last of it
back where it was. `:trash` lists everything deleted, the last first:
Enter or `r` restores, `x` deletes one for good and `:trash purge` empties
it, each asking first. Something whose folder has gone since comes back to
the root of its space; something whose space is in the trash too waits for
the space. The daily sweep purges what has waited WORKSPACE_TRASH_DAYS (30),
and only then are its files deleted.

While it waits, a trashed node is out of the tree, the search, the agenda,
the mails, the SFTP mount, the export and the readable files/ view, and a
shared document answers 404 until it comes back with the same link. `rm`
over the mount trashes too; a rename onto a taken name still replaces what
was there, so editor saves do not fill the trash.

web/src/lib/Help.svelte

@@ -41,7 +41,8 @@
     ['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)'],
+    ['x · dd', 'delete what has focus — the row in the list, the document in its pane — into the trash (asks y/N)'],
+    ['u', 'undo the last delete: it comes back out of the trash — :trash has the rest'],
     [':', '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'],