fix(mobile): hide RightRail below md breakpoint

v1.6 left the right-rail file browser visible on phones (~32px column
when collapsed). Wrap the RightRail render in <div class="max-md:hidden
contents"> inside RightRailForSession so it's hidden entirely below
the md (768px) breakpoint. The `contents` class keeps the wrapper
layout-transparent on desktop. No behavior change on desktop.

apps/web/src/App.tsx

@@ -26,7 +26,13 @@ function RightRailForSession({ sessionId }: { sessionId: string }) {
       .catch((err) => console.warn('RightRail: failed to fetch session', err));
   }, [sessionId]);
   if (!projectId) return null;
-  return <RightRail projectId={projectId} />;
+  // Hidden entirely below md breakpoint; mobile users get the file browser
+  // via the FileBrowserPane infrastructure if/when it lands in workspace panes.
+  return (
+    <div className="max-md:hidden contents">
+      <RightRail projectId={projectId} />
+    </div>
+  );
 }
 
 function MobileBackdrop() {