Set Up Dependency Injection

Background: Native [iOS SwiftUI+Combine / Android Jetpack Compose / both]
app using [architecture]. Right now dependencies are [created inline /
singletons everywhere / passed by hand]. Key dependencies: [e.g., API
client, database, auth service, analytics].

Use case: Design a dependency injection approach so I can swap real
implementations for mocks in tests and avoid a tangle of singletons.

Implementation details:
- iOS: compare manual constructor injection, an @Environment-based
  approach for SwiftUI, and a container library. Avoid heavyweight setup
  for a small app.
- Android: compare manual/constructor injection vs Hilt/Koin; show how
  scoping works (app vs screen scope).
- Dependencies should be defined behind protocols/interfaces, not
  concrete types
- Show how a view model receives its dependencies

Layer & dependencies: Composition should happen at the app's entry/
assembly point; lower layers receive dependencies, never reach for them.

Deliverable: A recommended DI approach with reasoning, the wiring code
for one real dependency end to end (definition → injection → use in a
view model), and how the same dependency is replaced with a mock in a
unit test.