fix(host): keep the child pane reachable on narrow viewports; surface truncated levels

The Miller row now scrolls horizontally with the child pane pinned into
view when its preview lands, so descent stays reachable when the dialog
is narrower than two fixed panes. The dialog also says when a visible
level was cut at the backend's complete-result bound (817's maxEntries)
instead of letting the tail of a huge directory go silently missing.

packages/host/directory-picker-browse/tests/client-flow.spec.tsx

@@ -19,6 +19,7 @@ const homeListing: DirectoryListing = {
   home: HOME,
   crumbs: [{ name: '/', path: '/', hidden: false }, { name: 'u', path: HOME, hidden: false }],
   entries: [{ name: 'Documents', path: `${HOME}/Documents`, hidden: false }],
+  truncated: false,
 }
 
 async function bench() {