website: move placeholders to src/assets for Vite base64 inlining, add LazyVideo component

Placeholders are now generated into src/assets/placeholders/ instead of
public/ so Vite's asset pipeline processes them. Since all are < 4KB,
Vite auto-inlines them as base64 data URIs via assetsInlineLimit — zero
extra HTTP requests for placeholders.

- Static imports resolve to data:image/png;base64,... at build time
- JSDoc on placeholder props documents why static imports are required
  (synchronous availability, Vite inlining) and warns against dynamic
  imports and public/ paths
- New LazyVideo component: same pixelated poster pattern, uses native
  <video preload="none" loading="lazy"> for zero-JS lazy loading

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