Image performance: serve video instead of GIFs

[tmshv]

Problems

1. images.unoptimized: true serves raw GIFs — next.config.js:5
   GIFs in public/images/ are up to 7MB. The repo already has mp4/webm conversions for most of them. Serve <video> with mp4/webm sources instead of <img> for animated content.

2. SwitchImage uses next/legacy/image — src/components/SwitchImage/index.tsx:2
   Migrate from next/legacy/image to next/image. The legacy component will be removed in a future Next.js version.