feat: font sourcing integration in scaffold

[jubscodes]

Split from #36.

Problem

Scaffold doesn't handle font installation systematically. Premium fonts have a manual integration path; Google Fonts vs Fontsource isn't a configurable choice.

Proposal

Support multiple font sources in `gsp-scaffold` and `gsp-project-build`:

• Google Fonts — `next/font/google` or `@fontsource/`
• Fontsource — self-hosted, privacy-friendly
• Local/premium fonts — guide for adding purchased fonts (e.g., Berkeley Mono, Söhne)

Scaffold handles installation based on source type. Next.js projects use `next/font` for automatic optimization.

Considerations

• Font licensing — only recommend openly licensed fonts by default; flag premium fonts as "bring your own"
• Performance — recommend variable fonts where available, subset for used characters
• Fallback stacks — every custom font needs a well-matched system font fallback

Related

• feat: premium typography — curated font pairings and personality matching #36 — original typography umbrella (closed when split)