Introduce ThemeService to manage themes and dark mode state,
including initialization, persistence, and server sync.
Add UserPreferencesService to handle API communication for
retrieving and saving user preferences with in-memory caching
to reduce network overhead.
Register both services in the DI container in Program.cs.
Enhance ThemeService with detailed documentation and introduce
ThemeOption record for selectable themes. Improve server-side
caching in UserPreferencesService for consistency and efficiency.
