Render contributor avatars in the people template when available

[lreading]

Summary

Use contributor avatar data in the people/contributor spotlight template when it is available from generated data.

The GitHub connector already records generated.contributors.authors[].avatar_url and falls back to a GitHub avatar URL when needed, but the people slide currently resolves contributor names and handles while rendering icon badges instead of avatar images.

Acceptance criteria:

• People cards can render a contributor avatar from generated.contributors.authors[].avatar_url when a spotlight entry matches by login.
• The fallback remains polished when no avatar is available.
• Template docs, screenshots, examples, and visual/test coverage are updated as needed.

Alternatives considered

Keep icon-only cards, but then docs and schema language should stop implying that avatars appear in the rendered spotlight.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure