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.tsx

@@ -252,6 +252,15 @@ export function DirectoryBrowser({ open, listDirectory, createDirectory, onOpen,
     const trail = crumbTrailRef.current
     if (trail !== null) trail.scrollLeft = trail.scrollWidth
   }, [crumbTail])
+  // On viewports too narrow for both fixed panes the Miller row scrolls;
+  // whenever a child preview lands, pin it into view the way the crumb tail
+  // pins — otherwise descent is unreachable on a phone-width window.
+  const millerRowRef = useRef<HTMLDivElement | null>(null)
+  const childPath = child?.path
+  useEffect(() => {
+    const row = millerRowRef.current
+    if (row !== null && childPath !== undefined) row.scrollLeft = row.scrollWidth
+  }, [childPath])
 
   if (!open) return null
   const twoPane = selected !== null
@@ -362,7 +371,7 @@ export function DirectoryBrowser({ open, listDirectory, createDirectory, onOpen,
         </div>
       </div>
       <div className={css.content}>
-        <div className={css.millerRow}>
+        <div className={css.millerRow} ref={millerRowRef}>
           {parent !== null && (
             <LevelColumn
               entries={parent.entries}
@@ -384,6 +393,11 @@ export function DirectoryBrowser({ open, listDirectory, createDirectory, onOpen,
           )}
         </div>
         {loading && <div className={css.status} role="status">{t('browser.loading')}</div>}
+        {/* The backend bounds a level at its complete-result limit; say so
+          * whenever a visible pane was cut instead of letting the tail of a
+          * huge directory go silently missing. */}
+        {(parent?.truncated === true || child?.truncated === true) && !loading
+          && <div className={css.status} role="status">{t('browser.truncated')}</div>}
         {error !== null && <div className={css.error} role="alert">{error}</div>}
       </div>
       <div className={css.footerBar}>