Build the Networking Layer

Background: Native [platform] app, [SwiftUI+Combine / Compose+Kotlin].
API: [REST / GraphQL], auth: [bearer token / other]. I want
[URLSession-only / Retrofit / Ktor / undecided].

Use case: Build a reusable, testable networking layer the whole app
shares.

Implementation details:
- A typed request/endpoint definition and JSON decoding (Codable /
  kotlinx.serialization / Moshi)
- Surface typed domain errors, not raw transport errors
- Bearer-token auth with automatic refresh on 401 and request retry
- Timeouts, retry-with-backoff for transient failures, and cancellation
  tied to the screen lifecycle
- iOS: expose async/await or Combine publishers; Android: suspend
  functions or Flow
- A protocol/interface seam so it's mockable in tests

Layer & dependencies: Lives in the data layer behind an interface; the
repository consumes it, the UI never sees URLs or status codes.

Deliverable: The API client interface, one concrete implementation, the
error type, the auth-refresh flow, and a worked example of mocking the
client in a unit test.