Add cross-browser e2e tests, field-limit cheat sheet, and docs

Verification:
- Add Playwright harness (e2e/) covering Chromium, Firefox, and WebKit plus
  Android/iOS mobile emulations. Smoke tests assert: default flyer renders
  with no JS/console errors, @media print hides app chrome and keeps the
  flyer, anonymous npub generation (WebCrypto), legacy /nostr/ redirect
  preserves query+hash, and event-lookup URLs show the loading state instead
  of the default flyer. All six projects pass.
- package.json/playwright.config.js wire up the projects and a managed static
  server; .gitignore excludes node_modules and Playwright output.

Docs:
- docs/flyer-field-limits.md: per-script character cheat sheet for each flyer
  field (single-line caps + multi-line guidance), measured by rendering each
  script at print size.
- README: add a Features section matching the shipping client and a
  cross-browser verification section; link the cheat sheet.
- AGENTS.md: document the e2e suite and the one-time WebKit Linux dep.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.gitignore

@@ -9,6 +9,11 @@ venv/
 # Node/browser tooling
 node_modules/
 
+# Playwright e2e output
+test-results/
+playwright-report/
+playwright/.cache/
+
 # Build and local output
 build/
 dist/