Locale-aware defaults: browser language list + US Letter/A4 paper size

Language inference:
- Walk navigator.languages (the full ordered preference list), not just
  navigator.language, and pick the first base language we support. A user who
  prefers e.g. Dutch then German now gets German instead of the English
  fallback. De-duplicates the default-language resolution into setDefaultText.

Paper size (US Letter + A4):
- Add A4 (210x297mm) alongside US Letter (8.5x11in). Sheet dimensions move to
  CSS custom properties (--sheet-w/--sheet-h) toggled by a paper-letter/paper-a4
  class on <html>; the tear-off column and QR codes stay fixed physical sizes
  and the content column flexes.
- @page size is injected by JS (id voxvera-page-size) since @page cannot read
  CSS variables across engines; the print block keeps margin:0 and a Letter
  default for the pre-JS state.
- Default paper is chosen from the browser locale's region subtag
  (en-US -> Letter, en-GB/de-DE -> A4) — "by location" without geolocation,
  falling back to Letter when no region is present. A manual override selector
  (topbar + viewer controls) persists in localStorage and wins over the default.
- Paper size is a local print/view preference and is never written into the
  Nostr event, so a flyer reprints on either size unchanged.
- Add paper_size/paper_letter/paper_a4 labels for all 14 languages.

Docs + tests:
- Spec gains a "Print paper size" section; AGENTS.md records the
  not-in-payload invariant; README adds a feature bullet; the field-limits
  cheat sheet gains Letter/A4 columns (re-measured per paper size).
- Playwright: add tests for navigator.languages inference, region-based paper
  default (A4 and Letter), and override persistence. 54 tests pass across
  Chromium, Firefox, WebKit, and Android/iOS mobile emulations.

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

docs/flyer-field-limits.md

@@ -9,7 +9,8 @@ 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:
+Capacity varies by **script** (not by individual language) and by **paper
+size**, for these reasons:
 
 1. The flyer body font is monospace (`Courier New`), so within a script every
    character takes the same width regardless of upper/lowercase.
@@ -17,19 +18,22 @@ Capacity varies by **script**, not by individual language, for three reasons:
    [`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.
+4. **Paper size.** A4 (210×297mm) is slightly narrower and taller than US
+   Letter (8.5×11in), so single-line fields hold ~2 fewer characters on A4
+   while the body holds a little more. Both sizes are shown below.
 
 ## 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).
+(codepoints), shown as **Letter / A4**.
 
-| 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 |
+| Languages | Title & Headline | Subtitle |
+|---|---|---|
+| Latin & Cyrillic & Hebrew — en, es, de, fr, pt, sw, tr, ru, he | ~28 / ~26 | ~32 / ~30 |
+| Arabic & Persian — ar, fa | ~28 / ~27 | ~38 / ~36 |
+| Devanagari — hi | ~36 / ~35 | ~51 / ~48 |
+| CJK — ja, zh | ~17 / ~16 | ~23 / ~21 |
 
 ## Multi-line fields (generous)
 
@@ -37,12 +41,12 @@ these are the limits people actually run into. Counts are characters
 (`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) |
+| Languages | Body (content) — Letter / A4 |
 |---|---|
-| Latin & Cyrillic & Hebrew | ~1,900 (a few hundred words) |
-| Arabic & Persian | ~1,850 |
-| Devanagari — hi | ~2,450 |
-| CJK — ja, zh | ~1,170 |
+| Latin & Cyrillic & Hebrew | ~1,900 / ~2,000 (a few hundred words) |
+| Arabic & Persian | ~1,850 / ~1,930 |
+| Devanagari — hi | ~2,450 / ~2,545 |
+| CJK — ja, zh | ~1,170 / ~1,220 |
 
 `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
@@ -55,7 +59,7 @@ 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
+  at true print size for both Letter (8.5×11in) and A4 (210×297mm). 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.

docs/nostr-static-client-spec.md

@@ -122,6 +122,36 @@ The client and schema helper must:
 - keep attachments unsupported in V1
 - avoid remote frontend dependencies
 
+## Print paper size
+
+The flyer supports two print sizes: **US Letter** (8.5×11in) and **A4**
+(210×297mm), covering every country whose language the client supports (Letter
+in North America and a few Latin American countries; A4 everywhere else,
+including the EU, Russia, China, Japan, and the Middle East).
+
+- Dimensions live in CSS custom properties `--sheet-w` / `--sheet-h`, switched
+  by a `paper-letter` / `paper-a4` class on `<html>` (`applyPaperSize` in
+  `nostr-client.js`). The tear-off column (`3.75in`) and QR codes (`1.25in`)
+  stay fixed physical sizes; the content column flexes to the sheet width.
+- The print `@page { size: … }` is injected by JS (an element with id
+  `voxvera-page-size`), because `@page size` cannot read CSS custom properties
+  across engines. The static `@media print` block keeps `margin: 0` and a
+  Letter default for the pre-JS state.
+- The default is chosen from the **region subtag** of the browser locale
+  (`preferredPaperSize` — e.g. `en-GB` → A4, `en-US` → Letter). This is "by
+  location" without geolocation, using the same signal as language. When no
+  locale carries a region, it falls back to Letter.
+- A user override (the paper-size selector) is stored in `localStorage`
+  (`voxvera_paper`) and wins over the region default.
+- **Paper size is a local print/view preference and is never written into the
+  Nostr event.** A flyer authored on Letter reprints on A4 for a European
+  viewer without changing the source event. The live fit-check protects
+  authoring on whichever size is active; switching to a narrower size after
+  authoring can ellipsis-truncate a maxed-out single-line field.
+
+Field capacity therefore varies by paper size; see
+[`flyer-field-limits.md`](flyer-field-limits.md).
+
 ## Localization
 
 Flyer defaults come from `site/locales.js`.