FileBrowser can report the folder it is showing

A chooser whose answer is a folder had no way to ask. Folders navigate rather than being pickable —
getting to a file requires it — so the only way to say "this one" is to be standing in it, and the
caller needs to know which one that is to put a button on it.

Reported from the component rather than threaded through the hook: the hook's job is the folder,
and a seventh positional argument is how a signature stops being readable.

src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/FileBrowserApp.tsx

@@ -1,3 +1,4 @@
+import { useEffect, useRef } from 'react';
 import { Breadcrumb } from './components/Breadcrumb';
 import { Toolbar } from './components/Toolbar';
 import { UploadProgress } from './components/UploadProgress';
@@ -35,6 +36,14 @@ type FileBrowserAppProps = {
    * handling reimplemented, all of it free to drift from the real browser.
    */
   onPickFile?: (file: PickedFile) => void;
+  /**
+   * The folder being shown, whenever it changes.
+   *
+   * For a chooser whose answer is a FOLDER rather than a file. Folders navigate rather than being
+   * pickable — getting to a file requires it — so the only way to say "this one" is to be standing in
+   * it, and the caller needs to know which one that is to put a button on it.
+   */
+  onPathChange?: (path: string) => void;
 };
 
 export const FileBrowserApp = ({
@@ -45,9 +54,18 @@ export const FileBrowserApp = ({
   panelId,
   pathParam,
   onPickFile,
+  onPathChange,
 }: FileBrowserAppProps) => {
   const fileBrowserManager = useFileBrowserApp(basePath, rootOverride, urlPath, panelId, pathParam, onPickFile);
-  const { handleNavigate } = fileBrowserManager;
+  const { handleNavigate, currentPath } = fileBrowserManager;
+
+  // Reported from here rather than threaded through the hook: the hook's job is the folder, and one
+  // more positional argument on a six-argument call is how a signature stops being readable.
+  const report = useRef(onPathChange);
+  report.current = onPathChange;
+  useEffect(() => {
+    report.current?.(currentPath);
+  }, [currentPath]);
 
   return (
     <div className="flex flex-col h-full overflow-hidden">