Upgrade Astro 5 -> 7 with Vite 8 (staged via Astro 6)

[chkry]

Staged framework upgrade landing on Astro 7.0.2 + Vite 8 (Rust compiler), keeping the unified() remark/rehype Markdown pipeline for output parity.

• astro 5.16 -> 6.4.8 -> 7.0.2; Shiki 4; @astrojs sitemap/rss/check bumped
• markdown: processor: unified() via @astrojs/markdown-remark 7
• import z from astro/zod (astro:content re-export deprecated)
• fonts moved from experimental to top-level; dropped preserveScriptOrder flag (now default); removed obsolete tailwindcss() @ts-ignore
• compressHTML: true to preserve v6 whitespace under the Rust compiler
• consolidate on pnpm: remove package-lock.json, add pnpm-workspace.yaml (allowBuilds for sharp/esbuild, verifyDepsBeforeRun: false), add .nvmrc
• CI/deploy: Node 20 -> 22, pnpm 10.11 -> 11.9
• repo-wide prettier format; eslint-disable for inline-script console

Verified: astro check 0 errors, lint/format clean, 34 pages, pagefind/OG/RSS/sitemap intact.

Description

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)

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)

Further comments

Related Issue

Closes: #