Add ThemeService and UserPreferencesService

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.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs

@@ -34,6 +34,10 @@ builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
 
+// UI Preferences & Theme
+builder.Services.AddScoped<UserPreferencesService>();
+builder.Services.AddScoped<ThemeService>();
+
 // DevExpress WASM
 builder.Services.AddDevExpressWebAssemblyBlazorPdfViewer();
 builder.Services.AddDevExpressWebAssemblyBlazorReportViewer();