⚡ Bolt: optimize scroll nav class mutations

[kaitoartz]

💡 What: Added a dirty check before updating nav-active classes.
🎯 Why: Unconditionally removing and adding classes on every frame forces the browser to evaluate style changes, causing unnecessary layout thrashing.
📊 Impact: Eliminates redundant DOM mutations when the active section hasn't changed, freeing up main thread CPU time during scroll.
🔬 Measurement: Verify scroll framerate and check for fewer layout recalculations in Chrome DevTools Performance tab.

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PR created automatically by Jules for task 4203594539398008611 started by @kaitoartz

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