feat(mobile): set up HTTP client for backend API calls

[GRACENOBLE]

Problem

The mobile app only communicates with Firebase. There is no HTTP client configured to call the Go backend (/api/v1/*), and no pattern for injecting the Firebase ID token into API requests.

Acceptance criteria

• Configure an OkHttp/Retrofit (or Ktor) client in a DI-friendly location (e.g. data/network/ApiClient.kt)
• Add an OkHttp interceptor that attaches the Firebase ID token as Authorization: Bearer <token>
• Define a BASE_URL build config field in build.gradle.kts (reads from local.properties or a build variant)
• Add a working example call: fetch /api/v1/me and display the result in the home or settings screen
• Document the pattern in mobile/docs/

[GRACENOBLE]

Implemented and merged via PR #65.