Add French locale and translation files

[Kalebtron1]

Why this matters

Expand the app to a third language and reduce English-only friction.

Problem

French users still see untranslated or partially translated UI.

Scope

• Add fr as a supported locale.
• Translate the main product flows first.
• Keep fallback behavior stable.

Setup Steps

• Clone the repository and run npm install at the repo root.
• Run npm run dev and switch the app to French.
• Run npm run build to confirm the locale changes do not break the app.

File Paths

• src/App.tsx
• src/pages/Login.tsx
• src/pages/Onboarding.tsx
• src/components/WalletSetupModal.tsx
• src/components/CreditSection.tsx
• src/components/ActivityList.tsx

Implementation guidance

• Reuse the existing i18n structure already in the app.
• Start with login, onboarding, wallet setup, credit, and history.
• Keep the translation keys aligned with the current English source strings.

Acceptance criteria

• PR includes the French locale and the translated core screens.
• Reviewer can switch to French and see the app update without missing-key crashes.
• The PR description names which screens were translated first.

Validation

• Switch locale to French and review the main screens.
• Confirm missing keys do not break the UI.
• Verify the translated copy still fits the current layouts.

PR requirements

• Include Closes #67.
• Include before/after behavior notes.

Complexity

• Medium (100 points)

[ironhood]

@ironhood has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Can I work on this before the wave end?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ironhood to this issue.