keys act on the focused pane, deletes ask y/N in place

dd, x, i, R, s, y and :mv used the open document whenever one was open,
so walking the list and pressing dd deleted the document beside it. They
now act on the row under the cursor while the list has focus. Deletes ask
an in-app y/N naming the node instead of confirm(), and the board no
longer lets tree keys through to the hidden list.

web/src/lib/Help.svelte

@@ -17,7 +17,7 @@
     ['o', 'new document here, straight into insert mode'],
     ['O', 'new folder here'],
     ['i · a', 'edit the open document'],
-    ['R', 'rename what is marked — document, folder or card'],
+    ['R', 'rename what is under the cursor — document, folder or card'],
     ['m / p', 'pick a node up, then drop it into the folder you walk to'],
     ['b', "open this space's board"],
     ['Esc · q', 'close the document'],
@@ -25,7 +25,7 @@
     ['n', 'clear the search'],
     ['s', 'toggle sharing for the document'],
     ['y', 'mint a new share link and copy it'],
-    ['x · dd', 'delete what is selected (asks first)'],
+    ['x · dd', 'delete what has focus — the row in the list, the document in its pane (asks y/N)'],
     [':', 'command line'],
     ['Ctrl-S', 'save, from anywhere'],
     ['?', 'this screen — in the editor too'],