[F2V] Improve drag and drop notice

[skjnldsv]

• While dragging in the header, if your cursor is hovering over the header, there should be some hover feedback, similar to when your cursor is over a subfolder. So the background of the header section can change to --color-primary-light and the font color to --color-primary for example.
• Since this is shown only while dragging, the message could be even more attention grabbing, so we could make the font bold

(ref. Gdrive's drop indication which is really attention grabbing)

Originally posted by @nimishavijay in #40674 (comment)