New media upload popup with pause, resume, and persistent visibility

Media uploads now run inside a persistent popup that floats above the dashboard so you can keep working while files transfer.

• The popup is visible on the media pages (such as Media and the media detail page) so you can navigate between them without losing upload progress; it is not shown on unrelated pages.
• Individual files can be paused, resumed, or removed from the queue without affecting the rest of the batch.
• Multiple audio and video files can be uploaded together, including pull-by-URL submissions alongside direct file picks.
• The popup stays open by default and only closes when you explicitly dismiss it, preventing accidental loss of uploads.
• Progress is reported per file, so retries pick up where they left off after a network hiccup.

Mobile gets a tailored layout so the queue is usable on small screens without obscuring the page underneath.