Update non-TypeScript JavaScript parsers

[joshka]

Summary

Updates the remaining non-TypeScript JavaScript dependencies that validated cleanly together:

• marked 15.x → 18.x
• remark-svgbob 1.x → 2.x
• toml 3.x → 4.x
• prettier-plugin-tailwindcss 0.6.x → 0.7.x

This intentionally leaves TypeScript and tree-sitter updates for separate follow-up work.

Why these together?

These affect formatting, markdown rendering, and TOML parsing. They are higher risk than pure support-tool updates, but they validated together without code changes or broad formatter churn.

I left out:

• typescript 6, because compiler diagnostics and Astro check behavior should be reviewed separately.
• tree-sitter / tree-sitter-rust, because tree-sitter@0.25.0 failed the native build under Node 24 locally: Node 24's V8 headers require C++20, but the package build did not use C++20 here.

Verification

• mise exec -- corepack pnpm format:check
• mise exec -- corepack pnpm test -- --run
• mise run build on the local hydrated-LFS testing stack
• astro preview + Playwright smoke checks across:
  • home page
  • svgbob-heavy docs pages
  • developer guide
  • tutorial pages with local images
  • v0.30 highlights
  • production search

One external GitHub camo image on /faq/ reports broken in Playwright, so the image assertion was scoped to local assets for this dependency validation.

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