fix: Lighthouse a11y and perf — aria, contrast, images, browserslist

[AndSanG]

Summary

• aria-hidden-focus — back-to-top button now sets tabIndex={-1} when aria-hidden="true", so it can't receive keyboard focus while invisible
• Color contrast — dark mode text in the skills marquee and experience card descriptions bumped from zinc-500 to zinc-400 (contrast ratio 4.35:1 → 7.96:1, clears WCAG AA 4.5:1 threshold)
• Image sizes — swift-logo.webp 512 px → 80 px (16 KB → 2 KB), tri.webp 330 px → 80 px (7.7 KB → 1.7 KB), about-me.webp recompressed at q=78 (146 KB → 113 KB)
• Legacy JS polyfills — added browserslist to package.json targeting Chrome/Edge/Firefox ≥ 100 and Safari ≥ 15.4; all polyfilled features (Array.prototype.at, Object.hasOwn, flat, flatMap, Object.fromEntries, String.prototype.trimEnd/trimStart) are native in these targets, dropping ~14 KB from the JS bundle

Test plan

• Run Lighthouse on the deployed site and verify accessibility violations are gone
• Confirm dark mode contrast looks correct in marquee and experience cards
• Verify navbar logos render crisply at 40 px on both 1× and 2× displays
• Check about-me photo quality in the More About section