feat(video-player-v2): add buffered range visual and style gradient to scrub bar

[kduncanhsu]

Summary

• Add buffered range visual indicator to the v2 video player scrubber bar, matching the existing v1 behavior
• Add a black-to-transparent gradient background behind the v2 video controls for better contrast against video content as per designs
• Move bufferedRange into sharedProps in DashViewer so both v1 and v2 controls receive it consistently

Changes

• TimeControlsV2.tsx: Accept bufferedRange prop, compute buffered percentage, and render a three-segment track gradient (played → buffered → unplayed) using custom color constants (rgb(127, 127, 127) for buffered,
  rgb(85, 85, 85) for unplayed)
• VideoControlsV2.tsx: Forward bufferedRange to TimeControlsV2
• VideoControlsV2.scss: Add linear-gradient(to top, rgba(0,0,0,1), rgba(0,0,0,0)) background
• DashViewer.js: Move bufferedRange into sharedProps (removes duplicate prop on v1 VideoControls)

Screenshots

Test plan

• Play a DASH video with the v2 player enabled and verify the scrubber shows three distinct segments: white (played), medium grey (buffered), dark grey (unplayed)
• Verify the buffered segment grows as the video loads ahead of the playhead
• Verify the gradient background fades from black at the bottom to transparent at the top behind controls
• Verify the v1 player still renders its buffered range correctly
• Run existing unit tests: TimeControlsV2, VideoControlsV2, DashViewer (all passing)

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