⚡ Bolt: Optimize section rendering with content-visibility

[ImChong]

💡 What: Applied content-visibility: auto and contain-intrinsic-size: auto 500px to the .section class in css/style.css.
🎯 Why: In a long single-page application like this portfolio, content far below the fold forces the browser to calculate layout and paint for the entire page on initial load. content-visibility: auto allows the browser to skip rendering for off-screen sections until the user scrolls near them, decreasing main thread blocking and accelerating the initial page render.
📊 Impact: Expected to reduce initial main thread layout and paint time significantly, especially on lower-end devices, improving time to interactivity.
🔬 Measurement: Verified the performance optimization locally. No layout breakage was detected. Captured screenshot and recording for code review, validating graceful degradation and intrinsic sizing works.

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PR created automatically by Jules for task 3755859976763749732 started by @ImChong

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