Add rendering regression check

[shigahi]

Summary

Closes #149.

• Test page: Worknot Render Check in the CME workspace. It has an inline database with table (plus a filtered tab), board, gallery, list, calendar and timeline views, a formula property, an uploaded image, a page cover, a PDF, a file attachment, a Mermaid code block, a table of contents with an anchor target, and a subpage.
• scripts/render-check.mjs (npm run check:render):
  • Generates a worker for that page from src/code.ts (through Vite, so it runs on any supported Node version) and runs it with wrangler dev.
  • Loads the page through it in Chromium and checks: the page renders, all six database views, rows and formula values, images, PDF/file blocks, Mermaid, embeds/bookmarks (skipped when the page has none), anchor links, opening a database item on the same domain, subpage navigation with back/forward, and no failing requests.
  • Every check also runs against notion.site. A failure that also happens there is reported as NOTION and doesn't fail the run, so Notion's own flakiness doesn't raise false alarms.
  • Exit codes: 0 = pass, 1 = regression through Worknot, 2 = the check could not run.
• Daily workflow: the Notion monitor workflow gets a render-check job on Node 24 (wrangler requires Node.js 22+). It opens or updates a render-regression issue on exit code 1.
• Baseline: baselines/notion-snapshot.json is refreshed for Notion client 23.13.20260925.1117, released today. Only the version and app bundle name changed, and the render check passes with it.
• Adds playwright as a dev dependency (npm audit: 0 vulnerabilities).

Verification

• Local run: all checks pass through the worker. database rows and formula is NOTION: since today's Notion client update, the table, gallery and list views show "No results" on notion.site itself too, and a worker built from the old master (fc1dacb) behaves the same.
• Negative test: a worker with JS rewriting disabled makes navigation and history FAIL, and the script exits 1.
• Setup failure (unusable port) exits 2.
• workflow_dispatch on this branch: run 36169736563. Both jobs succeeded with the same results as locally.
• Two earlier test runs opened false-alarm issues, both now closed: Rendering regression detected - 2026-09-25 #158 (Node 20 couldn't start wrangler) and Rendering regression detected - 2026-09-25 #159 (the Mermaid check didn't wait for lazy rendering). Both causes are fixed in this PR.

Notes

• The test page must stay published. X/GitHub embeds can be added under its "Embeds" heading; the embed check currently skips.
• .nvmrc still pins Node 20, which reached end of life in April 2026 and can no longer run wrangler dev / wrangler deploy (wrangler 4.131 requires Node.js 22+). Worth a separate update.