⚡ bolt: use transform instead of width for scroll progress bar

[dttdrv]

💡 what: updated .scroll-progress css and script.js to animate transform: scaleX() instead of width.
🎯 why: animating width during scroll events triggers layout thrashing (forced synchronous layouts) and repaints on every frame, which hurts rendering performance. gpu-accelerated transforms bypass layout recalculations.
📊 impact: eliminates layout thrashing during scroll, ensuring consistent 60fps scrolling and reducing main thread blocking.
🔬 measurement: use chrome devtools performance tab to observe layout shift reductions and decreased rendering times during scroll. verify visual behavior is unchanged.

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PR created automatically by Jules for task 14820758040364259713 started by @dttdrv

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