ui: tweak media player style and behavior

[CyanVoxel]

Summary

This PR makes a few design and QoL tweaks to the media player:

• Simplified layout and style
• Black hover overlay replaced with gradient behind controls
• Added deadzone around player controls where clicking won't pause/unpause
• Added more generous margins between controls
• Fixed clicking to jump far into unbuffered media
• Misc code cleanup

Known (preexisting) issue: Clicking to jump to a presumably unbuffered section of media does now allow for continuous dragging until clicked again.

Before:

After:

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable