Show buffered video range in fullscreen controls

[Simonh98]

Summary

Adds a buffered-range indicator to the fullscreen video seek bar.

The playback position remains shown in the accent color, while the lighter
background indicates the time range currently buffered by the native video
element.

Implementation

• Reads the active buffered range from HTMLMediaElement.buffered
• Updates the indicator on progress, timeupdate, waiting, canplay, and durationchange
• Keeps seek and playback position handling unchanged
• Handles multiple buffered ranges after seeking

Validation

• npm.cmd run build passes
• Tested with the LG webOS TV fullscreen player