feat: add French locale and translation files

[Dhozii]

Overview

This PR adds French as a fully supported locale and translates the main product flows first — login, onboarding, wallet setup, credit, and activity/history — while keeping English as a stable fallback. It reuses the existing i18n structure, registers fr in the locale config, and adds the French translation file so reviewers can switch the app to French without missing-key crashes.

Related Issue

Changes

🌐 French Locale and Core Screen Translations

• [ADD] src/i18n/locales/fr.ts

  • Adds French translations for the core screens: Login, Onboarding, Wallet Setup, Credit Section, and Activity List.
  • Keys stay aligned with the current English source strings and reuse the existing i18n namespace structure.
  • Includes fallback-safe values so missing translations do not crash the UI.

• [MODIFY] src/i18n/config.ts

  • Registers fr as a supported locale in the app's i18n configuration.
  • Keeps existing locale fallback behavior stable.

• [MODIFY] src/components/LanguageToggle.tsx

  • Adds French as a selectable language option in the UI language switcher.

Verification Results

npm install
✅ Dependency install succeeds

npm run dev
✅ French locale selectable from LanguageToggle
✅ Core screens render in French without missing-key crashes
✅ English fallback behavior unchanged

npm run build
✅ Production build completes successfully

Acceptance Criteria	Status
PR includes French locale and translated core screens	✅ fr registered and Login/Onboarding/WalletSetup/Credit/ActivityList translated
Reviewer can switch to French and see the app update without missing-key crashes	✅ Locale switch works; missing keys fall back safely
PR description names which screens were translated first	✅ Login, Onboarding, Wallet Setup, Credit Section, Activity List
Before/after behavior documented	✅ Before: French users saw partial/missing UI; After: core flows are translated with stable English fallback

Closes #67

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