feat: upgrade to Astro 6 with self-hosted Fonts API

[vairi]

Summary

Upgrades the entire monorepo from Astro 5.18 to Astro 6.1.7 and adopts the new Fonts API for self-hosted, optimized web fonts.

What changed

Astro 6 upgrade

• "astro": "^5" → "^6" in all 4 site package.json files
• No code-level breaking changes hit us (no ViewTransitions, no Astro.glob, no legacy content collections)
• Node 22.15 meets the v6 requirement (22.12+)

Fonts API adoption (the big improvement)

Before: Each site loaded fonts via @import url('https://fonts.googleapis.com/...') in global.css. This meant:

• Runtime requests to Google's servers on every page load
• User data sent to a third party (GDPR concern)
• No font optimization or fallback generation

After: Fonts are configured in astro.config.mjs via the Astro 6 Fonts API:

import { defineConfig, fontProviders } from 'astro/config';

export default defineConfig({
  fonts: [
    {
      provider: fontProviders.google(),
      name: 'Inter',
      cssVariable: '--font-heading',
      weights: [400, 500, 600, 700, 800, 900],
    },
  ],
});

This means:

• Fonts are downloaded at build time and self-hosted from your own domain
• Zero runtime requests to fonts.googleapis.com
• Automatic optimized fallback fonts with matching metrics (no layout shift)
• Better privacy — no user data sent to third parties
• Preload links generated automatically

Scaffolder updated

scripts/new-site.sh now generates the Fonts API config in astro.config.mjs per preset instead of writing @import lines in CSS.

Font config per preset

Preset	Heading font	Body font
Corporate	Inter	Inter
SaaS	Sora	Inter
Warm	Playfair Display	Source Sans 3

Test plan

• bun install clean (Astro 6.1.7 installed)
• bun run build — all 4 sites build successfully
• No TypeScript errors
• No deprecated API usage found (no ViewTransitions, Astro.glob, or legacy collections)
• CI passes
• Visual verification that fonts render correctly on demo sites after deploy
• Lighthouse check for font performance improvement

Risk

Low. Astro 6 is a clean upgrade for this codebase — we weren't using any of the removed/changed APIs. The font change is the only user-visible difference and it should look identical (same fonts, same weights) while being faster.