Avatar component: switch to next/image and keep the fallback chain #52

Problem

components/ui/avatar.tsx:22 renders <img src={chain[0]}>. The component tries a list of sources in order (chain), dropping to the next on error. Because it's shared, fixing it here improves every page that uses <Avatar>.

Done when

• It renders next/image's <Image>, with width/height taken from the component's size prop.
• The fallback still works. Test by pointing a member at a GitHub username that doesn't exist: the next source (or the initials) must appear, not a broken image.
• npx eslint components/ui/avatar.tsx shows no no-img-element.
• npm run build passes.

Size: ~10–20 lines.

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