Linux: Export for web doesn't allow selecting existing folder, neither from outside nor inside

I tried to re-export the same project twice with Export for web... and quickly noticed this:
1. the pop-up reopens *inside* the last folder where you exported: OK
2. inside: the Save button is greyed out since you are inside a folder and not selecting a folder nor entering a new folder name manually
3. you can type a folder name but then you'll create a new *subfolder* inside the initial export folder
4. outside: you can go up one level, select the initial export folder again and click Save but then it enters the folder again
5. only way is to create a new folder with a *different name* OR delete the existing folder

Recently, I found out about Export story.json only... which removes the issue of folder selection and allows me to keep any CSS override.

Still, for users who actually want a brand new export e.g. because they messed up with CSS previously and want to revert the changes to default CSS, it would be nice to allow selecting an existing folder (from outside and inside). Other game engine project selection popup generally allow this.

Start by reproducing the Export for web flow and compare selecting an existing folder from inside and outside the current export folder. Trace the folder-selection UI used by that flow; done means an existing folder can be selected in both cases without entering it again or forcing a new name.

Written by the indexing model from the issue text.