Migrate ReflexGame UI to Compose and StateFlow

[juandiana]

Summary

This PR migrates the app from XML/View-based UI to Jetpack Compose, moves UI state observation from LiveData to Flow/StateFlow, and aligns theming/state structure with Compose-first practices while preserving the existing look/behavior.

High-level changes

• Replaced the View/XML screen implementation with Compose UI in MainScreen.kt (grid, HUD, button, and game-over title).
• Added MainScreenRoute(viewModel) that collects state with collectAsStateWithLifecycle, while keeping MainScreen(...) stateless and previewable.
• Added @Preview for the main screen.
• Renamed MainUiState to MainScreenState for naming consistency.
• Moved SquareState into the new UI state file and removed old ui/model state files.
• Migrated MainViewModel from LiveData to MutableStateFlow/StateFlow.
• Replaced nullable/live-data copy patterns with explicit _uiState.value = _uiState.value.copy(...) updates.
• Renamed the square interaction callback to onSquareTouched across the codebase.
• Refactored chronometer responsibility to ViewModel:
  • timer start/stop lifecycle is handled in ViewModel
  • elapsed time is formatted in ViewModel via DateUtils.formatElapsedTime(...)
  • UI now only renders the formatted text
• Extracted Compose theme artifacts into dedicated files:
  • theme/Color.kt
  • theme/Theme.kt
  • theme/Spacing.kt
  • theme/Type.kt
• Removed no-longer-used View/XML resources:
  • deleted activity_main.xml and item_square.xml
  • deleted colors.xml and dimens.xml
• Simplified themes.xml for Compose usage (kept app theme + status bar color only).
• Removed no-longer-used LiveData helper classes (Event, EventObserver) and old adapter (SquaresAdapter).
• Updated Gradle/dependencies for Compose-first setup:
  • enabled buildFeatures.compose = true
  • added Kotlin Compose plugin alias
  • added kotlin entry to the version catalog and referenced it from the Compose plugin
  • set Kotlin to 2.3.10
  • switched to Compose BOM + Material3 + tooling dependencies
  • removed obsolete view-system dependencies
• Hardened MainViewModelFactory to validate modelClass before casting.

Validation

• ./gradlew :app:assembleDebug
• ./gradlew :app:lintDebug

Both pass locally.