fix: PromptChoiceScreen subtitle showed target language instead of primary

[TrainTravel]

Summary

PromptChoiceScreen rendered the heading and subtitle as identical strings — e.g. both lines read "Comment voulez-vous commencer ?" for any user on the default fr/en pair. The user hit this in screenshot.

Root cause

The subtitle used a Translations object with mis-keyed values:

// BEFORE — broken
<h2>{t({ fr: 'Comment voulez-vous commencer ?', en: 'How would you like to start?', es: '...' }).primary}</h2>
<p>{t({
  fr: 'Comment would you like to start?',         // ← ENGLISH under fr key
  en: 'Comment voulez-vous commencer ?',          // ← FRENCH under en key
  es: 'Comment voulez-vous commencer ?',          // ← FRENCH under es key
}).secondary}</p>

For target=fr primary=en, .secondary looks up the en key and got the French value — so the subtitle rendered French again, duplicating the heading visually.

Fix

Compute the heading once and use both .primary (target lang) and .secondary (primary lang) from the same Translations object:

const heading = t({
  fr: 'Comment voulez-vous commencer ?',
  en: 'How would you like to start?',
  es: '¿Cómo quieres empezar?',
  ja: 'どのように始めますか？',
  'zh-Hans': '你想怎么开始？',
  'zh-Hant': '你想怎麼開始？',
});
// ...
<h2>{heading.primary}</h2>
{heading.secondary !== heading.primary && <p>{heading.secondary}</p>}

The conditional render avoids showing the subtitle when both sides happen to resolve identically (e.g. target=ja primary=en both fall back to the en value if ja: were missing — same pattern as PR #39's bilingual() dedupe).

While here, added ja / zh-Hans / zh-Hant keys to all 7 t() calls on this screen — same pattern as PR #40's Tier 1 chrome sweep.

Test plan

• npx tsc --noEmit — clean
• npx vitest run — 188 passed / 189 (only pre-existing useJournal startFreeWrite)
• Manual: open PromptChoice with target=fr primary=en → heading reads French, subtitle reads English ("How would you like to start?")
• Manual: target=ja primary=en → heading reads Japanese, subtitle reads English
• Manual: target=zh-Hant primary=zh-Hans → heading reads Traditional, subtitle reads Simplified (not identical strings)

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