fix(files): Correctly validate new node name

fix(files): Correctly validate new node name

* Resolves https://github.com/nextcloud/server/issues/45409

This includes two fixes:

1. The name in the "new node" dialog is correctly selected (e.g. `file.txt` only `file` is selected by default), to allow quick naming

2. `@nextcloud/files` functions for filename validation are used, this allows to use new Nextcloud 30 capabilities (e.g. reserved names)

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>