feat(phone-app): migrate Settings screen to Jetpack Compose Material 3

[hungify]

What to build

Migrate the Settings screen (SettingsActivity & activity_settings.xml) to a full declarative Jetpack Compose Material 3 screen.

Acceptance criteria

• Create SettingsScreen.kt using Material 3 Scaffold, TopAppBar, SingleChoiceSegmentedButtonRow / Radio options for default travel mode selection, and OutlinedTextField inputs for driving/walking vibration thresholds.
• Connect with SettingsViewModel via collectAsStateWithLifecycle() for preferences state and error validation.
• Show instant input validation errors and save confirmation via Material 3 SnackbarHost.
• Update SettingsActivity to extend ComponentActivity and render SettingsScreen via setContent { WatchNavigatorTheme { ... } }.
• Verify functionality via SettingsViewModelTest and full build verification.

Blocked by

• feat(phone-app): setup Jetpack Compose BOM, Kotlin Compose compiler, and Material 3 theme #41