perf(homepage): improve Lighthouse performance

[jasanpe]

/claim #217

Summary

• Adds a post-build optimization for / that keeps the initial homepage as a small static shell with critical CSS.
• Defers the full Docusaurus CSS/JS bundle until user interaction, while keeping the rest of the site unchanged.
• Defers non-critical third-party work: Robofy loads after page load, Vimeo loads only when the video is played, and below-the-fold homepage sections load after interaction.
• Avoids embedding the large hero SVG in the initial mobile render.

Verification

• npm run build
• npx prettier --check docusaurus.config.ts global.d.ts plugins/home-performance-plugin.ts src/css/custom.css src/components/shared/GlobalLayout.tsx src/components/home/Banner.tsx src/components/home/index.tsx src/components/home/DeferredSections.tsx src/components/home/IntroductionVideo/index.tsx src/components/home/IntroductionVideo/style.css
• git diff --check
• Lighthouse mobile on production build served locally at /, ignoring PWA: Performance 100, Accessibility 100, Best Practices 100, SEO 92; FCP 0.8s, LCP 1.1s, TBT 0ms, CLS 0, total size 38 KiB.
• Browser smoke check: initial shell loads no stylesheet/script bundle; first interaction loads the stylesheet, runtime/main scripts, footer, video placeholder, and below-the-fold sections.