i18n(pt): complete and ship Brazilian Portuguese

- Translate the remaining 1,868 strings (pt-BR, as in #909) and review the
  contributor entries: HUD codes BRG/SPD/RANGE stay as in the other
  languages, sentence-case layer and scene names, Space key named as on the
  keyboard, short PREV/NEXT/ON/OFF chips, and the CLDR many form (exact
  millions take «de») on every plural.
- Register pt: any pt tag (pt-BR, pt-PT) maps to it; the picker shows
  «Português (Brasil)».
- Codes, units and product names identical to English go to
  same-as-english.json.
- docs/translations/pt.md: style, plurals, units and glossary.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/i18n/i18n.test.mjs

@@ -34,6 +34,8 @@ test('normalizeLocale folds regional variants and rejects unshipped tags', () =>
   assert.equal(normalizeLocale('ru-RU'), 'ru');
   assert.equal(normalizeLocale(' RU '), 'ru');
   assert.equal(normalizeLocale('en_GB'), 'en');
+  assert.equal(normalizeLocale('pt-BR'), 'pt');
+  assert.equal(normalizeLocale('pt_PT'), 'pt');
   assert.equal(normalizeLocale('xx'), null);
   // Explicit script tags take precedence over regional defaults.
   assert.equal(normalizeLocale('zh'), 'zh-CN');
@@ -142,6 +144,20 @@ test('tn() picks English and Russian plural forms', () => {
   assert.match(label(21), / 21 КЛЮЧ(?![А-ЯЁ])/);
 });
 
+test('tn() picks the Portuguese one, many and other forms', () => {
+  const label = (count) =>
+    tn(
+      count,
+      'POWER UP · {count} KEY WAITING',
+      'POWER UP · {count} KEYS WAITING',
+    );
+  setLocale('pt');
+  assert.equal(label(0), 'ATIVAR · 0 CHAVE PENDENTE');
+  assert.equal(label(1), 'ATIVAR · 1 CHAVE PENDENTE');
+  assert.equal(label(3), 'ATIVAR · 3 CHAVES PENDENTES');
+  assert.equal(label(1_000_000), 'ATIVAR · 1000000 DE CHAVES PENDENTES');
+});
+
 test('t() context separates identical English with different meanings', () => {
   assert.equal(t('CLEAR', { context: 'action' }), 'CLEAR');
   setLocale('ru');