(#59) Add settings screen and functionality

[bfalls]

Fixes #59

• Added a gear icon entry point on the home banner and wired MainActivity to toggle between home and a new full-screen Settings screen that refreshes the home view after updates.
• Built a reusable LocationPickerPane composable to share the manual/device location UI between onboarding and settings while preserving the existing city search flow.
• Introduced a persisted banner size preference and threaded it through settings, view model state, and the sky renderer to scale sun and moon drawings based on the selected size.
• Added an AppThemeMode enum to user settings and persisted it in DataStore with a dedicated preference key so theme choices survive restarts.
• Surfaced theme controls on the Settings screen and wired the view model to update and broadcast the selected mode immediately for recomposition.
• Applied the selected theme at the app root via AppCompatDelegate and refreshed the light/dark color palettes and splash resources to align splash, onboarding, and home surfaces with the chosen mode.