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]>

docs/flyer-field-limits.md

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+# Flyer Field Length Cheat Sheet
+
+How much text fits in each flyer field, by script.
+
+VoxVera does **not** impose a fixed character count. The editor measures the
+rendered flyer live and rejects a keystroke the moment a field would overflow
+the printable sheet (you'll see an inline "too long" error under that field).
+So the real limit is enforced for you as you type — the numbers below are
+practical guidance for drafting copy, sizing translations, or prompting an LLM
+to write flyer text that will fit on the first try.
+
+Capacity varies by **script**, not by individual language, for three reasons:
+
+1. The flyer body font is monospace (`Courier New`), so within a script every
+   character takes the same width regardless of upper/lowercase.
+2. Per-script letter-spacing policy (`ls-full` / `ls-cjk` / `ls-none`, see
+   [`nostr-static-client-spec.md`](nostr-static-client-spec.md#letter-spacing-by-script)).
+3. Glyph width differs by script — CJK glyphs are full-width (≈2× a Latin
+   character), Devanagari fallback glyphs are narrower, etc.
+
+## Single-line fields (hard caps)
+
+**Title**, **Subtitle**, and **Headline** are single-line: they never wrap, so
+these are the limits people actually run into. Counts are characters
+(codepoints).
+
+| Languages | Title | Headline | Subtitle |
+|---|---|---|---|
+| Latin & Cyrillic & Hebrew — en, es, de, fr, pt, sw, tr, ru, he | ~28 | ~28 | ~32 |
+| Arabic & Persian — ar, fa | ~28 | ~28 | ~38 |
+| Devanagari — hi | ~36 | ~36 | ~51 |
+| CJK — ja, zh | ~17 | ~17 | ~23 |
+
+## Multi-line fields (generous)
+
+**Body** (content), **Link message** (`url_message`), and **Footer**
+(`footer_message`) wrap across multiple lines, so they hold far more and are
+bounded only by the remaining height of the sheet.
+
+| Languages | Body (content) |
+|---|---|
+| Latin & Cyrillic & Hebrew | ~1,900 (a few hundred words) |
+| Arabic & Persian | ~1,850 |
+| Devanagari — hi | ~2,450 |
+| CJK — ja, zh | ~1,170 |
+
+`url_message` and `footer_message` share the lower portion of the sheet and can
+hold roughly 1,200–4,800 characters depending on script, but for a readable
+flyer keep the link message to a sentence and the footer to a short line.
+
+The **URL** field is a link, not body copy — keep it short both because it's
+shown verbatim on the flyer and because it becomes the main QR code (shorter
+URLs scan more reliably).
+
+## Notes and caveats
+
+- **Approximate.** These come from rendering each script in a headless browser
+  at true print size (8.5in × 11in, content column 4.75in). The
+  Latin/Cyrillic numbers (`Courier New`) are the most reliable; numbers for
+  Hebrew, Arabic, Persian, Devanagari, and CJK depend on the system fallback
+  font, which can differ slightly between devices and from print output.
+- **The app is authoritative.** If the editor accepts your text it fits; if it
+  rejects it, shorten it. Treat this sheet as a target, not a contract.
+- **Redaction markup counts.** Wrapping text in `~~double tildes~~` renders it
+  struck-through; the four `~` characters count toward the field length even
+  though they don't appear on the flyer.
+- **Re-measuring.** If the flyer layout, font, or letter-spacing policy
+  changes, these numbers should be re-derived by rendering each script at print
+  size and binary-searching each field for the largest run of characters that
+  does not overflow the field target or the sheet.