feat(mobile): establish error state and retry UI pattern

[GRACENOBLE]

Problem

There is no established pattern for displaying errors and allowing users to retry. Each screen will handle this ad-hoc.

Acceptance criteria

• Create a reusable ErrorScreen composable that accepts a message and an optional onRetry callback
• Define a sealed UiState<T> class (Loading, Success(data: T), Error(message: String)) in a shared location
• Show an example of the full UiState lifecycle in one screen/ViewModel pair
• Document the pattern in mobile/docs/architecture.md
• Add instrumented tests for ErrorScreen

[GRACENOBLE]

Implemented and merged via PR #66.