Implement basic onboarding navigation & logic

[opatry]

The onboarding flow should guide the user through initial setup when preferences are not yet defined.

Flow:

1. App start
   → Check if preferences are set
   → If not, show Welcome screen
2. On “Continue”
   → Show Preferences screen to select:
   • Volume unit (ml or oz)
   • Notifications enabled (true/false)
   • Notification period (in hours)
3. Once submitted
   → Navigate to Main screen (placeholder)

Tech notes:

• Domain logic (use cases) in :preferences
• UI flow logic can live in a new :onboarding module
• Entry point & navigation in :h2go-app
• Use androidx.navigation.compose:2.9.x
• Reuse VolumeUnit enum and UserPreferences domain model

Deliverables:

• Domain layer: use case(s) for saving/loading preferences
• Presentation layer: ViewModel(s) per screen
• Compose UI screens with state management
• Navigation logic setup using Compose Navigation
• UI tests for Composables (runComposeUiTest)
• Navigation tests (recommend testing user flows via NavHostController assertions)

Testing:

• Unit tests for domain & ViewModels
• Composable tests for UI behavior
• Suggest test strategy for nav graph (e.g. dependency-injected NavController)