feat(native): Android tablets get the shortcuts, two panes meet at a foldable's fold

The Android shell now reports keyCommands and windowLayout, so the large
screen layout no longer assumes an iPad:

- The keyboard host hands the shortcuts to the app whenever the shell has
  the keyCommands feature, whatever the platform. Before it also required
  iOS, so an Android tablet with a keyboard answered the keys in the page
  and never listed them in the system's shortcuts.
- useWindowFold follows the layout the Android shell reports (feature
  windowLayout). When a vertical fold separates the window, a foldable half
  open like a book, MobileShell marks it with data-fold and its place, and
  the leading pane (the trip's plan, a tab's summary, a list beside its
  detail) ends at the fold while the trailing one starts there, on either
  reading direction. A flat fold that does not separate stays one screen.
- The split size class starts at 840pt, Android's expanded width, instead
  of 900pt, so a foldable unfolded on its side (about 841dp) gets two panes
  like an iPad does. An 11" iPad upright (834pt) stays a single pane. The
  address screen of the native shell follows the same width.
- TrekShell exports NativeWindowLayout for it.

Also: the sheet's width and anchor types and the collab board type stay
private to their modules, which nothing else imports (knip).

client/native-shell/shell.css

@@ -983,7 +983,7 @@ a.linkbtn {
    centred in the other half, the way a split sign-in page uses the width.
    Upright, the photo band grows with the screen instead of stopping at a
    phone's 440px. Logical sides, so Arabic mirrors it. */
-@media (min-width: 900px) and (min-height: 600px) and (orientation: landscape) {
+@media (min-width: 840px) and (min-height: 600px) and (orientation: landscape) {
   body::after {
     inset: 16px auto 16px 16px;
     inset-block: 16px;