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/src/client/DirectoryBrowser.module.css

@@ -43,12 +43,16 @@
  * so the edit zone to the right never leaves the bar. */
 /* The Miller columns keep their own row so a status/error line below never
  * competes with the fixed column widths for horizontal space. */
+/* A narrow viewport shrinks the dialog below two fixed panes; the row
+ * scrolls horizontally (the effect pins the child pane into view) so
+ * descent never hides behind the Modal's clipping. */
 .millerRow {
   display: flex;
   align-items: stretch;
   flex: 1 1 0;
   min-height: 0;
   gap: 20px;
+  overflow-x: auto;
 }
 
 .crumbTrail {