Add file size estimation and download progress display

[Curucail]

This pull request adds estimated file size display and download progress features to the track switcher UI, enhancing user feedback during audio track loading #1 . The changes include new logic to estimate total download size before activation, live progress updates during download, and UI adjustments to display this information in the overlay.

User Experience Improvements:

• Added a new overlay element (#overlaytext) styled in trackswitch.css to display status messages such as estimated download size and live download progress. [1] [2]

Download Progress and File Size Estimation:

• Introduced logic to estimate and display the total download size before track activation, using HEAD or GET requests to retrieve file sizes for each track source. [1] [2]
• Implemented live download progress tracking for each track, aggregating progress and displaying it in the overlay during downloads. [1] [2]