chore: migrate to Astro 7

[lmammino]

Summary

Migrates the site from Astro 5.16.16 to Astro 7.0.6 (crosses two majors, 5 → 6 → 7), following the official upgrade guides for v6 and v7.

Dependency bumps

• astro 5.16.16 → 7.0.6, @astrojs/react → 6.0.1, @astrojs/sitemap → 3.7.3
• astro-expressive-code + @expressive-code/plugin-collapsible-sections → 0.44.0 (Astro 7 peer support)
• @lucide/astro 0.522 → 1.23 (Astro 7 peer support)
• tailwindcss + @tailwindcss/vite → 4.3.2 (Vite 8 peer support), @tailwindcss/typography → 0.5.20
• new: @astrojs/markdown-remark (see below), mdast-util-to-hast (devDep, type augmentation for the admonitions plugin)

Code/config changes for breaking changes

• Markdown pipeline: Astro 7 defaults to the new Sätteri pipeline, which would silently drop our remark-based admonitions. The config now explicitly uses processor: unified({ remarkPlugins: [...] }) from @astrojs/markdown-remark. (Porting admonitions to Sätteri could be a future follow-up.)
• Fonts: experimental.fonts graduated → top-level fonts (same shape).
• compressHTML: true pinned: the new 'jsx' default strips whitespace between inline elements, which our templates rely on (e.g. icon + label in BlogLayout).
• Zod 4: z.string().url() → z.url(); z imported from astro/zod.
• tsconfig: dropped baseUrl — with it set, Astro 7's tsconfig-alias plugin stats every bare specifier and crashes (ENAMETOOLONG) on the base64 data: font URI inside swiper's CSS. paths resolve relative to the tsconfig without it, so @components/@lib/@images aliases are unaffected.
• Footer GitHub icon: lucide 1.x removed brand icons; Footer now uses the existing local ui/social/github.astro (whose <title> wrongly said "LinkedIn", fixed).
• engines.node >= 22.12.0 added (Astro 7 requirement; CI already runs Node 24).

Sanity testing (v5 baseline build vs v7 build)

• ✅ Page inventory identical (15 pages, all html/xml/txt routes)
• ✅ Structural per-page diff identical: admonition counts, expressive-code block counts, astro-island counts, TOC presence, font CSS vars/preloads, <title>s, and all h2/h3 heading IDs (no anchor/SEO churn)
• ✅ In-page anchor validator: every href="#x" resolves on both builds
• ✅ rss.xml byte-identical modulo lastBuildDate (12 items), sitemaps valid XML with same 14 URLs, JSON-LD blocks byte-identical
• ✅ Image markup: all 157 srcsets identical; only diffs are fetchpriority="auto" no longer emitted (browser default) and 12 markdown images gaining an inert empty srcset="" (Astro 7's no-upscale rule; browsers fall back to src)
• ✅ Headless-Chrome screenshots of /, /blog/, /blog/nodejs-http-request/, /404: fonts, hero image, hydrated counters, admonitions, code blocks, TOC, avatars all render correctly (404's WebGL area is blank in headless capture on the v5 baseline too, i.e. not a regression)
• ✅ pnpm build, typecheck, lint, format:check all green

Remaining known cosmetic diffs: lucide 1.x icon SVG markup, expressive-code copy button gained an aria-live region (a11y improvement).

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