webui: fix folder creation

[neooriginal]

before

after

[gemini-code-assist]

Code Review

This pull request effectively fixes the layout and positioning issues with the folder creation and deletion dialogs by using React Portals and improving the CSS for centering and overflow handling. The use of a mounted state to handle server-side rendering hydration is also correct.

However, I've identified a significant accessibility issue in both dialogs: they do not trap keyboard focus, which is a requirement for modal dialogs to be accessible. I've left comments on both the FolderDialog and DeleteFolderDialog components with details on how to address this. Addressing this is important for providing an inclusive user experience for keyboard and screen reader users.