Localize auth emails and formatter tooltip copy to BM and Chinese

[AlaskanTuna]

The 28/04/26 i18n pass shipped English, Bahasa Melayu, and Simplified Chinese across the app and the PDF export, but its own closing note records two surfaces left behind: the auth email templates and the tooltip copy in frontend/src/lib/formatters.ts. Given the product's stated audience is Malaysian homeowners making a financial decision, an English-only verification email is the very first thing a Malay-preferring user sees. The email half only becomes possible once task 8 brings the templates in-repo — until then they were locked in a Supabase dashboard.

Scope: backend/src/emails/, backend/src/services/emailService.ts, frontend/src/lib/formatters.ts, frontend/src/locales/{en,ms,zh}/

Depends on: #7

Checklist

• Add a locale variant per email template (confirm, reset, email-change, invite) and select on a stored user-locale preference, falling back to English
• Persist the user's locale at signup so the verification email — sent before the user ever has a session — can be localised
• Extract the remaining hardcoded strings from frontend/src/lib/formatters.ts into the existing locale files
• verify: signing up with the UI in BM delivers a BM verification email; same for zh
• verify: no hardcoded user-facing English string remains in formatters.ts
• verify: pnpm test green and pnpm typecheck clean
• verify: pnpm dlx impeccable detect <changed frontend files> reports 0 unwaived findings

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_{Priority: Medium · Phase 12 task 13 of the maintainer's migration plan.}