feat(i18n): add i18next with French and Portuguese translations

[OG-wura]

Close #19

PR Description

Summary

Closes #ISSUE_NUMBER — Internationalize StepFi for French and Portuguese
speakers in emerging African markets.

What's Changed

Dependencies

• i18next, react-i18next, expo-localization

New Files

File	Purpose
src/locales/en.json	English translations (~130 keys)
src/locales/fr.json	Complete French translations
src/locales/pt.json	Complete Portuguese translations
src/locales/i18n.ts	i18next config — async device detection, SecureStore persistence, isRTL() for future Arabic, formatCurrency()/formatDate() locale-aware helpers
hooks/useTranslation.ts	Custom hook exposing t, changeLanguage, currentLanguage, isRTL

Modified Files

File	Changes
app/_layout.tsx	Wait for i18n init before rendering
app/(tabs)/_layout.tsx	Dynamic tab labels via useTranslation()
app/(tabs)/index.tsx	All strings → t() calls + locale-aware dates
app/(tabs)/loans.tsx	All strings → t() calls
app/(tabs)/calendar.tsx	All strings → t() calls + localized weekday/month arrays
app/(tabs)/reputation.tsx	All strings → t() calls + localized tips/tiers
app/(tabs)/settings.tsx	All strings → t() + language picker modal (EN/FR/PT)
app/(auth)/sign-in.tsx	All onboarding slide text, tier labels, wallet names → t()
app/(auth)/role-select.tsx	Role titles, descriptions, pills → t()
app/(auth)/register.tsx	Labels, placeholders, button text → t()
components/reputation/ReputationProgressWidget.tsx	Widget strings → t()

Acceptance Criteria

• All UI text translatable
• French translation complete
• Portuguese translation complete
• Language persists on app restart (SecureStore / localStorage)
• npx expo export --platform web passes

Testing

1. Run npx expo start — app loads in device-detected language
2. Go to Settings → tap Language → pick Français
3. Verify all screens display in French
4. Kill and restart app — language persists
5. Repeat for Português