refactor: t() / bilingual() positional → object form

[TrainTravel]

Summary

Convert the t() and bilingual() helpers in LanguageContext.tsx from positional (fr, en, es, zhHans?, zhHant?) arguments to a single Translations object argument. All 350 call sites swept programmatically via an AST-based codemod (TypeScript compiler API).

// BEFORE
t('Bonjour', 'Hello', 'Hola', '你好', '你好')
bilingual('Vide-tête', 'Brain Dump', 'Volcado mental')

// AFTER
t({ fr: 'Bonjour', en: 'Hello', es: 'Hola', 'zh-Hans': '你好', 'zh-Hant': '你好' })
bilingual({ fr: 'Vide-tête', en: 'Brain Dump', es: 'Volcado mental' })

Return shapes do NOT change: t() still returns { primary, secondary }, bilingual() still returns "target / primary". No user-visible change. This is pure dev-ergonomics work — pulled out of the original Language Pair PR (#25) for a clean diff per the discussion in memory/project_language_pair.md.

Why now

• Adding a 6th supported language (Japanese, planned) costs nothing once Translations is an object — just add a new optional key. With the positional form, every call site would have had to grow a 6th arg.
• The 5% of call sites with template literals or variable refs (t(badge.fr, badge.en, badge.es)) are sed-unsafe but AST-safe — the codemod handles them automatically.
• Removes a class of silent ordering bugs where swapping two positional args compiles fine but ships broken UI.

Test plan

• npx tsc --noEmit — clean
• npx eslint . --ext .ts,.tsx — 11 errors / 27 warnings, identical to origin/main baseline; zero new issues
• npx vitest run — 155 passed / 156 total (only pre-existing useJournal startFreeWrite fails; confirmed unchanged on main)
• All 25 LanguageContext tests pass under the new signatures
• Codemod script kept at scripts/codemod-t-object-form.mjs for future similar refactors
• CI E2E will run on push — flaky baseline pre-existed
• Manual: load the app, swap languages, verify chrome strings still render in correct lang

Codemod notes

• Tooling: TypeScript compiler API (ts.createSourceFile + AST walk + getStart/getEnd text-range edits)
• Per-file scope detection — only files that destructure t/bilingual from useLanguage() are touched
• Already-object calls are skipped (idempotent)
• Source text preserved verbatim per argument (template literals, accented chars, escape sequences, ternaries, variable refs all flow through unchanged)
• 3 call sites in LanguageContext.test.tsx use result.current.t(...) member-access — the AST walker's identifier matcher doesn't catch these; updated manually

🤖 Generated with Claude Code