feat: dedupe bilingual() identical sides + add CTA ja/zh translations

[TrainTravel]

Summary

Fixes the "Write today / Write today" rendering when the active language pair has no translation for either side (e.g. target=ja and primary=zh-Hant, where both fall back to en via the Translations fallback chain).

Two changes

1. bilingual() dedupes identical sides (structural fix)

One-line change in src/contexts/LanguageContext.tsx:

return tgt === prm ? tgt : `${tgt} / ${prm}`;

When targetLang and primaryLang resolve to the same string, render once instead of "X / X". Silent improvement for every bilingual() call site across the app — no API change, no semantic change for pairs that legitimately differ (e.g. fr / en).

This means chrome strings without ja: / zh-*: keys silently improve too: "Brain Dump / Brain Dump" becomes "Brain Dump" when both sides fall back to en.

2. CTA gets explicit Japanese + Simplified Chinese + Traditional Chinese (per-call site fix)

The most visually prominent bilingual call on HomeScreen. "Write today" / "Write another" now render in the target language:

• ja: 今日書きましょう / もう一度書きましょう (polite, inviting register matching the encouragement tone of the button)
• zh-Hans: 今天写日记 / 再写一篇
• zh-Hant: 今天寫日記 / 再寫一篇

Other chrome anchors (Brain Dump, Thought Garden, French journaling practice, More tools, etc.) are NOT translated in this PR — the dedupe handles their visual ugliness, and a translation-pass PR will fill in real ja / zh-* keys for them.

Test plan

• npx tsc --noEmit — clean
• npx vitest run — 191 passed / 192 (only pre-existing useJournal startFreeWrite)
• 3 new tests in LanguageContext.test.tsx:
  • same-string-both-sides → renders once
  • different-strings → keeps the pair
  • all keys provided → no false dedupe; actual ja/zh-Hant pair renders
• Manual: open Home with target=ja primary=zh-Hant — CTA reads 今日書きましょう / 今天寫日記 instead of Write today / Write today
• Manual: existing pairs (target=fr primary=en) — CTA still reads Écrire aujourd'hui / Write today (no regression)
• Manual: any bilingual anchor (Brain Dump) with target=ja — renders Brain Dump once instead of Brain Dump / Brain Dump

Native review caveat

I picked the Japanese and Chinese phrasings myself. For 今日書きましょう I went with polite-encouraging register matching the button's vibe (not curt 今日書く, not heavy 今日の日記を書きましょう). A native pass via iTalki would catch any awkwardness — same recommendation as the Japanese prompts from PR #36.

Out of scope (follow-up translation pass)

• Adding ja: / zh-*: keys to other bilingual() call sites (Brain Dump, Thought Garden, etc.)
• Same for t() call sites in journaling-flow screens
• formatDate locale handling for primary=zh-* / target=ja (currently falls back to en-US)

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