Add bundle-size budget check to CI

Summary

The repo has no bundle-size budget or regression test in CI. The Vercel previews show size only on click-through; contributors have no automated warning when a commit fattens the bundle, and no historical size trend for comparison.

What goes wrong

• Unnecessary imports (e.g., rehype-raw was carried as a dep after PR #992 stopped using it; react-plotly.js lingers pending migration in #669; react-syntax-highlighter lingers pending #773) accumulate silently.
• Large dependencies can be added to package.json without any indicator of their bundle impact.
• Route-level code splitting regressions (e.g., a shared chunk suddenly containing a 500 KB vendor blob) are caught only during live slow-load reports from users.

Related active issues that would benefit from this (making them observable and prioritizable):

• #669 — Remove react-plotly.js
• #773 — Replace react-syntax-highlighter with Prism

Suggested fix

Add one or both:

1. Bundle-size budget CI check using size-limit or @next/bundle-analyzer:

# .github/workflows/pr.yaml (new job)
bundle-size:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - uses: oven-sh/setup-bun@v2
    - run: bun install --frozen-lockfile
    - run: bun run build
    - uses: andresz1/size-limit-action@v1
      with:
        github_token: ${{ secrets.GITHUB_TOKEN }}

…with per-route budgets in size-limit.config.js.

2. Lighthouse CI on the Vercel preview URL for perf + bundle metrics, posting a PR comment with the deltas.

Start with budgets based on current sizes (First Load JS per route from next build output); ratchet down over time.

Severity

Low — nice-to-have, becomes higher ROI once #669 / #773 / MarkdownFormatter dedupe (#990) land and you want to lock in the wins.

Relates to

#669, #773, #990.

Start by reading .github/workflows/pr.yaml and the build scripts in package.json, then inspect the First Load JS sizes reported by next build. The issue suggests either a size-budget check with size-limit.config.js or Lighthouse CI, so first determine which approach fits the existing workflow. Done means CI reports bundle-size changes against defined per-route budgets.