perf(player): optimize video scrubbing

[kcng0]

Summary

• keep seek feedback updates out of the video widget subtree
• pause while horizontal scrubbing and resume after the single final seek
• make the progress slider preview locally and seek once on release
• preserve existing playback behavior across Player, Viewer, and Visible preview

Verification

• flutter analyze targeted player files and tests
• flutter test video controls, Player Back, and Viewer Back tests
• git diff --check

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