feat(analysis): localize bill tooltip copy and make number formatting locale-aware

[AlaskanTuna]

Closes #12 — the formatter/tooltip half. The auth-email half landed in #37.

Two genuinely English-only surfaces in a trilingual app

1. BILL_TOOLTIPS / NEM_TOOLTIPS were hardcoded English in lib/formatters.ts — twelve explanatory strings about TNB tariff components and NEM credit flow, rendered through <InfoTooltip> next to labels that were translated. A Malay or Chinese user saw a localized label with an English explanation.

Moved into analysis.json under billBreakdown.tooltips and billBreakdown.nemTooltips, translated for ms and zh, and routed through t(). Only BillBreakdown.tsx consumed them, so the change is contained. Key structure verified identical across locales (305 keys each).

2. Both Intl.NumberFormats were pinned to 'en-MY' — so currency and number grouping ignored the user's language entirely, even though LOCALE_TO_INTL already existed for exactly this and maps zh to zh-Hans-MY.

setFormatterLocale() now rebuilds them, called from LocaleProvider on mount and on every change. A setter rather than a per-call argument because these formatters have ~100 call sites; threading a locale through all of them would be a far larger diff than the behaviour warrants.

Proof it actually varies:

en: RM 1,234,567.89  |  9,876.5 kWh
ms: RM 1,234,567.89  |  9,876.5 kWh
zh: MYR 1,234,567.89 |  9,876.5 kWh

Incidental extraction, and why it was necessary

Importing LOCALE_TO_INTL from i18n.ts broke AnalysisPage.test.tsx: i18n.ts calls i18n.init() at module scope, so any importer drags that in, and a test that mocks i18n was suddenly loading the real one.

Rather than duplicating the map and inviting drift, the locale constants (SUPPORTED_LOCALES, SupportedLocale, DEFAULT_LOCALE, LOCALE_STORAGE_KEY, LOCALE_TO_INTL, isSupportedLocale) moved to a side-effect-free lib/locales.ts. i18n.ts re-exports them, so every existing importer is unaffected.

Correction to an earlier claim

#37's description said "nothing persists User.locale yet, so production still sends English." That was wrong. useLocale.tsx:83 already calls authClient.updateUser({ locale: next }) and reads it back on mount, and config/auth.ts passes it to the email renderers. The localized-email chain has been live end to end since #37 merged.

Verified

pnpm --filter frontend exec tsc --noEmit 0 errors · 216 frontend tests pass · Prettier clean · no BILL_TOOLTIPS/NEM_TOOLTIPS references remain.

https://claude.ai/code/session_01URDpaxhXyPLWnUQtvowNyM

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