feat: j/k scroll the open document, and import loose files

With a document open and not being edited, j/k scroll the reading pane,
gg/G jump to its top and bottom, and Ctrl-D/Ctrl-U page it. h still backs
out to the list, where the same keys walk the rows again.

The import pickers offered a folder or a zip and nothing else, so a single
file could not be imported at all. The second picker now takes any files;
the server already handled them.

web/src/lib/Help.svelte

@@ -3,8 +3,9 @@
   import { workspace } from './store.svelte'
 
   const keys: [string, string][] = [
-    ['j / k · ↓ ↑', 'move down / up the list'],
-    ['gg / G', 'first / last row'],
+    ['j / k · ↓ ↑', 'move down / up the list — scroll the open document'],
+    ['gg / G', 'first / last row, or top / bottom of the document'],
+    ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
     ['Enter · l', 'open a document, or enter a folder'],
     ['h', 'up one level, or back out of a document'],
     ['o', 'new document here, straight into insert mode'],