chore: upgrade Astro to v6.1.2

[davlgd]

Description

Upgrades Astro from 5.16.6 to 6.1.2, following the official Astro v6 Migration Guide.

Dependency bumps

Package	From	To	Reason
astro	^5.16.6	^6.1.2	Major upgrade (Vite 7, Zod 4, Shiki 4, Node ≥ 22.12)
@astrojs/rss	^4.0.14	^4.0.18	Required for Zod 4 compatibility (was breaking the build on /rss.xml with z.function(...).returns is not a function)
@astrojs/sitemap	^3.6.0	^3.7.2	Required for Zod 4 compatibility
@shikijs/transformers	^3.20.0	^4.0.2	Aligns with Shiki 4 bundled in Astro 6

Configuration changes

• astro.config.ts
  • Moved fonts out of experimental — the Fonts API is now stable in v6.
  • Removed experimental.preserveScriptOrder — preserved source order is the new default in v6, so the flag has been removed from the experimental API; dropping it keeps the current behavior.
  • Removed the obsolete @ts-ignore on the tailwindcss() Vite plugin: the upstream issue (withastro/astro#14030) is resolved with Vite 7 support in Astro 6.
• src/content.config.ts
  • Replaced the deprecated z re-export from astro:content with an import from astro/zod, as recommended by the migration guide.

Types of changes

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Documentation Update (if none of the other choices apply)
• Others (any other types not listed above) — dependency upgrade / migration

Checklist

• I have read the Contributing Guide
• I have added the necessary documentation (if appropriate)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected) — Node.js ≥ 22.12 is now required at build time

Further comments

Build verification

With local Node.js 24 LTS

✅ pnpm install              – clean
✅ pnpm sync                 – content synced, types generated
✅ pnpm exec astro check     – 0 errors / 0 warnings / 0 hints (50 files)
✅ pnpm run build            – 43 pages built
✅ Pagefind                  – search index generated (16 indexed pages, 2218 words)
✅ Sitemap                   – generated
✅ pnpm lint                 – clean

Breaking changes reviewed

• Node.js ≥ 22.12.0 — verified.
• Vite 7.0 — no custom Vite plugins affected; tailwindcss() works without the previous @ts-ignore workaround.
• Zod 4 — content collection schemas validated; @astrojs/rss and @astrojs/sitemap bumped to versions that already use Zod 4.
• Shiki 4 — @shikijs/transformers aligned to v4.
• Legacy content collections removed — already on the Content Layer API.
• Astro.glob() / <ViewTransitions /> removed — not used in source code.

Out of scope (suggested follow-ups)

These are stable v6 capabilities that AstroPaper could benefit from but are intentionally not included in this PR to keep the migration reviewable:

• security.csp — built-in Content Security Policy with automatic hashing of bundled scripts/styles. A natural fit for a blog template; would need a small allowlist if Giscus / analytics are enabled.
• prerenderConflictBehavior: "error" — fails the build when two routes generate the same prerendered URL (e.g. a post slug colliding with a tag).

Related documentation

• Astro v6 release blog
• Official v6 migration guide

Related Issue

Closes: #626

[satnaing]

Thanks for putting this together.
AstroPaper has been rewritten and upgraded for Astro v6.
Thus, closing this for now.
Really appreciate the effort on the migration work. ✌🏼