feat: set up API client and HTTP layer with auth interceptors

[Josue19-08]

🔖 Title

Set up API client and HTTP layer with automatic JWT handling

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📄 Description

All existing hooks (`use-sign-in`, `use-create-account`, `use-invest`) use `setTimeout` simulations or `console.log` instead of real API calls. There is no HTTP client configured in the project. This is the foundational piece that every other backend integration depends on.

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✅ Tasks to complete

• Install and configure `axios` (or `ky`) as the HTTP client
• Create `src/lib/api-client.ts` with base URL from `EXPO_PUBLIC_API_URL` env variable
• Add request interceptor to attach `Authorization: Bearer ` header
• Add response interceptor to handle 401 errors: auto-refresh token via `POST /auth/refresh`, then retry original request
• Create `src/lib/token-storage.ts` using `expo-secure-store` to persist `accessToken` and `refreshToken` securely
• Create typed service modules: `auth.service.ts`, `loans.service.ts`, `merchants.service.ts`, `liquidity.service.ts`, `notifications.service.ts`
• Add `EXPO_PUBLIC_API_URL` to `.env.example` and contributing guide
• Add unit tests for the interceptor logic

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📚 Documentation/context for AI

(This link never should removed)
https://github.com/TrustUp-app/TrustUp-API/tree/main/docs

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🗒️ Additional notes

• All API response shapes are documented in `TrustUp-API/docs/api/endpoints.md`
• Use TypeScript interfaces to type all API responses — no `any`
• The refresh token endpoint is `POST /auth/refresh` (body: `{ refreshToken }`)
• On refresh failure (expired refresh token), clear storage and redirect user to sign in
• Default base URL for local dev: `http://localhost:4000\`

[willy-de7]

@willy-de7 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'd love to work on this issue please, thank you

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