closing a file no longer sends you home

Open ~/Tests, double-click a file, edit it, save, close the pane — and the browser
was back at ~. Reported from a real session, not found by reading.

`onCloseViewer` was `setSearchParams({})`, which wipes the WHOLE query string. The
folder lives in `?path=`, so closing the viewer deleted where you were along with
what was on top of it. It also discarded anything `useGlobalQueryString` had put
in the URL.

The rule was already written down one file away, in
`useFileBrowserApp.setViewerParams`: "`path` is not one of them — it is where you
are, not what is on top of it — so it survives." Only the OPEN path honoured it.
Close now deletes the ephemeral keys by name, which is exactly what
`onCloseEphemeral`, `onCloseEphemeral2` and `onCloseChat` directly below it have
always done — it was the single outlier among four handlers.

Checked the other exits before calling it fixed: there is one `onClose` wired to
the viewer, every other handler already used the targeted form, and the
mount-cleanup effect deletes the same keys off `prev` rather than replacing the
string. This was the only wholesale wipe in the repo.

tsgo clean, frontend builds, 817 pass / 7 fail unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/officerdev/src/hooks/useFileViewerPanels/useFileViewerPanels.tsx

@@ -51,7 +51,27 @@ export const useFileViewerPanels = (): EphemeralPanels | null => {
         ? viewerWithEphemeralLayout
         : singleViewerLayout;
 
-  const onCloseViewer = useCallback(() => setSearchParams({}), [setSearchParams]);
+  /**
+   * Close the viewer and everything stacked on it — and NOTHING else.
+   *
+   * This was `setSearchParams({})`, which wiped the whole query string. That took `path` with it, so
+   * closing a file you had opened inside a folder dropped you back at home: open ~/Tests, view a file,
+   * close it, and the browser is at ~. It also discarded whatever `useGlobalQueryString` had put there.
+   *
+   * The intent was already written down in `useFileBrowserApp.setViewerParams` — "`path` is not one of
+   * them, it is where you are, not what is on top of it, so it survives" — but only the OPEN path
+   * honoured it. Deleting the ephemeral keys by name is the same rule applied to close, and it is what
+   * `onCloseEphemeral` directly below has always done.
+   */
+  const onCloseViewer = useCallback(
+    () =>
+      setSearchParams((prev) => {
+        const next = new URLSearchParams(prev);
+        EPHEMERAL_KEYS.forEach((k) => next.delete(k));
+        return next;
+      }),
+    [setSearchParams],
+  );
 
   const onCloseEphemeral = useCallback(
     () =>