feat(desktop): hover-reveal the file-browser sidebar too

The reveal mechanism already lives in the shared Pane primitive — the
right rail just opts in with hoverReveal. Its content renders
unconditionally, so (unlike the chat sidebar) it needs no extra
content-visibility gating.

apps/desktop/src/app/desktop-controller.tsx

@@ -848,6 +848,7 @@ export function DesktopController() {
     <Pane
       defaultOpen={false}
       disabled={!chatOpen}
+      hoverReveal
       id="file-browser"
       key="file-browser"
       maxWidth={FILE_BROWSER_MAX_WIDTH}