fix(website): upgrade to Astro v6 and Starlight 0.38.x

[bryan-thompsoncodes]

Summary

Upgrades the website to Astro v6 and Starlight 0.38.x to resolve the build error caused by Dependabot PR #692.

Problem

Dependabot PR #692 bumped @astrojs/starlight from 0.37.7 to 0.38.3, but Starlight 0.38.x requires Astro v6 (dropping v5 support). This caused:

[vite] The requested module 'astro/zod' does not provide an export named 'locales'

The locales export is an Astro v6 API that Starlight 0.38.x depends on internally. No code changes in our repo reference it — it's inside Starlight's own integration code.

Changes

• astro: ^5.18.1 → ^6.0.0 (required by Starlight 0.38.x)
• @astrojs/starlight: ^0.37.7 → ^0.38.3
• @astrojs/react: ^4.4.2 → ^5.0.0 (Astro v6 compatible line)
• starlight-links-validator: ^0.14.3 → ^0.23.0 (Zod 4 compatible — Astro v6 ships Zod 4)
• Updated pnpm-lock.yaml

No code changes needed

The website's content.config.ts already uses the Content Layer API (docsLoader/docsSchema), and astro.config.mjs required no changes. The upgrade was purely dependency versions.

Verification

• ✅ pnpm run build — 145 pages built successfully
• ✅ pnpm run checks — lint, format, types, astro check, spelling all pass
• ✅ pnpm run test — 13 tests pass (vitest)

Notes

• Astro v6 requires Node 22.12.0+ (CI already uses Node 22)
• Astro v6 includes Vite 7, Zod 4, and Shiki 4 as dependency upgrades
• This supersedes Dependabot PR chore(deps): bump @astrojs/starlight from 0.37.7 to 0.38.3 in the website-framework group across 1 directory #692 (which only bumped Starlight without the required Astro upgrade)

Closes #692