website: add pixelated image placeholders with fade-in transition

Build-time script generates tiny 32px-wide versions of all images in
public/ using sharp. At runtime, PixelatedImage component renders these
with CSS image-rendering: pixelated (nearest-neighbor / point sampling)
for a crisp mosaic effect, then fades in the real image on load.

- Auto-discovers images in public/, skips placeholder- prefixed files
- Mtime check skips regeneration when placeholder is newer than source
- Callback ref handles cached images where onLoad fires before mount
- Explicit z-index prevents placeholder from covering the real image
- Runs automatically as part of pnpm build in website package

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