test: keep zh locale keys aligned

[cat0825]

Summary

• Add the missing Simplified Chinese dataLoading.loadPlan.loadedCount_plural translation.
• Add a unit test that keeps the Chinese locale bundle's leaf keys aligned with English, preventing future fallback strings from slipping into the Chinese UI.

Related to #337

Verification

• npx vitest run tests/frontend/unit/app/i18nLocales.test.ts
• npx prettier --check src/i18n/locales/zh/dataLoading.json tests/frontend/unit/app/i18nLocales.test.ts
• git diff --check

[cat0825]

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