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/Rail.svelte

@@ -83,6 +83,7 @@
       ],
       [
         { icon: 'upload', label: 'Upload files into this folder', keys: [':upload'], run: () => hooks.pickTreeFiles(false) },
+        { icon: 'trash', label: 'The trash — what was deleted', keys: [':trash'], run: () => runCommand('trash') },
         ...(workspace.marked.length
           ? [{ icon: 'drop' as const, label: `Drop ${dropping(workspace.marked)} here`, keys: ['p'], run: () => workspace.guard(() => workspace.drop()), on: true }]
           : []),