Add CultureService and localization support

Added a new `CultureService` to manage application culture and
localization. The service supports retrieving and setting the
application culture, storing it in `localStorage`, and initializing
it based on stored values, browser settings, or a default fallback.

Registered `CultureService` in the dependency injection container
and added localization support in `Program.cs` using
`builder.Services.AddLocalization()`.

EnvelopeGenerator.ReceiverUI/Program.cs

@@ -26,6 +26,7 @@ builder.Services.AddScoped<DocReceiverElementService>();
 builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
+builder.Services.AddScoped<CultureService>();
 
 // Localization services
 builder.Services.AddLocalization();

EnvelopeGenerator.ReceiverUI/Services/CultureService.cs

@@ -0,0 +1,74 @@
+using System.Globalization;
+using Microsoft.JSInterop;
+
+namespace EnvelopeGenerator.ReceiverUI.Services;
+
+/// <summary>
+/// Service for managing application culture/localization.
+/// </summary>
+public class CultureService
+{
+    private readonly IJSRuntime _jsRuntime;
+    private const string CULTURE_KEY = "AppCulture";
+
+    public CultureService(IJSRuntime jsRuntime)
+    {
+        _jsRuntime = jsRuntime;
+    }
+
+    /// <summary>
+    /// Gets the list of supported cultures.
+    /// </summary>
+    public static CultureInfo[] SupportedCultures { get; } = new[]
+    {
+        new CultureInfo("de-DE"),
+        new CultureInfo("en-US"),
+        new CultureInfo("fr-FR")
+    };
+
+    /// <summary>
+    /// Sets the application culture and stores it in localStorage.
+    /// </summary>
+    public async Task SetCultureAsync(string culture)
+    {
+        if (!SupportedCultures.Any(c => c.Name == culture))
+            throw new ArgumentException($"Culture '{culture}' is not supported.", nameof(culture));
+
+        await _jsRuntime.InvokeVoidAsync("localStorage.setItem", CULTURE_KEY, culture);
+    }
+
+    /// <summary>
+    /// Gets the stored culture from localStorage.
+    /// </summary>
+    public async Task<string?> GetCultureAsync()
+    {
+        try
+        {
+            return await _jsRuntime.InvokeAsync<string?>("localStorage.getItem", CULTURE_KEY);
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    /// <summary>
+    /// Initializes the culture from localStorage or browser settings.
+    /// </summary>
+    public async Task<CultureInfo> InitializeCultureAsync()
+    {
+        var storedCulture = await GetCultureAsync();
+        
+        if (!string.IsNullOrEmpty(storedCulture) && 
+            SupportedCultures.Any(c => c.Name == storedCulture))
+        {
+            return new CultureInfo(storedCulture);
+        }
+
+        // Fallback to browser culture or default
+        var browserCulture = CultureInfo.CurrentCulture.Name;
+        var matchedCulture = SupportedCultures.FirstOrDefault(c => c.Name == browserCulture);
+        
+        return matchedCulture ?? SupportedCultures[0]; // Default to German
+    }
+}