Reduce React Doctor diagnostics in FilesView (#1271)

* Reduce React Doctor diagnostics in FilesView

* fix: address Greptile review feedback — use Base UI initialFocus and restore useMemo for displayedContent

- Replace useEffect-based dialog focus with Base UI Popup's initialFocus
  prop to avoid race with built-in focus management
- Drop unused no-op onOpenAutoFocus/onCloseAutoFocus from dialog wrapper
- Restore useMemo for displayedContent (200K char slice) to avoid
  per-render string allocation for large files
- Update DirectoryExplorerDialog to use initialFocus={false}

packages/ui/src/components/session/DirectoryExplorerDialog.tsx

@@ -707,7 +707,7 @@ export const DirectoryExplorerDialog: React.FC<DirectoryExplorerDialogProps> = (
     <Dialog open={open} onOpenChange={onOpenChange}>
       <DialogContent
         className="flex w-full max-w-xl flex-col gap-0 overflow-hidden p-0 sm:max-h-[80vh]"
-        onOpenAutoFocus={(event) => event.preventDefault()}
+        initialFocus={false}
       >
         <DialogHeader className="px-5 pb-2 pt-5">
           <div className="flex items-start justify-between gap-4">