✅(e2e) add test to compare generated PDF against reference template

[Ovgodd]

Purpose

Add a stable PDF export regression test that fails whenever the exported PDF for a full, complex document changes.

1. `npx playwright test __tests__/app-impress/doc-export.spec.ts --update-snapshots -g "full document" --project=chromium`
   -> refresh the reference PDF whenever we intentionally change the export output.
2. `npx playwright test __tests__/app-impress/doc-export.spec.ts -g "full document" --project=chromium`
   -> CI (and local runs without --update-snapshots) will compare the PDF to the reference
      and fail on any byte-level difference once the dynamic metadata has been sanitized.

Proposal

• Seed a doc with a fixed title and a prebuilt BlockNote/Yjs content (content.txt) via the REST API (PATCH /documents/:id/), using X-CSRFToken from Playwright cookies.
• Export to PDF on Chromium, sanitize dynamic metadata (dates, IDs, font prefixes),
  then compare bytes against a snapshot PDF.
• Document how to maintain the snapshot (--update-snapshots vs. normal run) directly in the test.
• Add print-datauris.js to generate PNG/JPG/SVG data URL helpers and update content.txt
  so the reference document includes embedded images via data URLs.