feat: i18n agency page and RoiCalculator via agencyPage namespace

[Gracecom1]

Summary

Complete i18n implementation for the agency white-label LP and its RoiCalculator component.

• All ~50+ hardcoded Japanese strings moved to messages/{locale}.json under the agencyPage namespace
• Full English translations provided (en.json)
• Remaining 10 locales populated with Japanese fallback keys
• Zero hardcoded UI strings remaining in agency/page.tsx and RoiCalculator.tsx

Changes

• src/app/[locale]/agency/page.tsx — Refactored to use getTranslations("agencyPage") for all section headings, plan data, steps, comparison table, FAQs, and CTA
• src/components/agency/RoiCalculator.tsx — Refactored to use useTranslations("agencyPage") for all labels and result text
• messages/*.json (12 locale files) — Added complete agencyPage namespace with structured plan/comparison/faq data

Related

• PAR-10 — pages-index.md source of truth
• PAR-28 — parent task