⚡ bolt: use css transform for scroll progress bar

[dttdrv]

💡 what: changed the .scroll-progress bar animation to use transform: scalex() instead of width. updated both script.js calculations and styles.css transitions.
🎯 why: animating width during high-frequency events like scrolling forces the browser to recalculate layout (reflow) on the main thread for every frame. using transform offloads the animation to the gpu compositor thread, avoiding layout thrashing and ensuring a smooth 60fps.
📊 impact: eliminates main thread layout reflows during scroll, resulting in significantly smoother animations, especially on lower-end devices.
🔬 measurement: tested locally using python server and playwright to verify that scalex updates correctly from 0 to 1 as the user scrolls to the bottom of the document. a visual check confirms the bar behaves identically to before.

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PR created automatically by Jules for task 16888047236284543025 started by @dttdrv

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