Define the State Management Approach

Background: Native [platform] app, [SwiftUI+Combine / Compose], using
[architecture]. Screens have this kind of state: [describe—e.g., loading
/loaded/error, form input, paginated lists, shared session state].

Use case: Establish one consistent state-management approach so every
screen handles state the same way.

Implementation details:
- Single source of truth and unidirectional data flow
- A standard pattern for async screen state (idle / loading / loaded /
  error) instead of scattered booleans
- iOS: where @State, @StateObject/@ObservedObject, @Observable, and
  Combine publishers each belong
- Android: StateFlow / collectAsStateWithLifecycle, and how UI events
  flow back up
- How shared/global state (auth/session, theme) is exposed without
  making everything a singleton

Deliverable: A documented state pattern with a reusable screen-state
type, an example view model / state holder using it, and guidance on
when state belongs to a screen vs. is shared app-wide.