Add localization support for multi-language UI

This commit introduces comprehensive localization support for the application, enabling dynamic language switching and localized UI elements. Key changes include:

- Added `.resx` files for German (de-DE), English (en-US), and French (fr-FR).
- Updated Razor components to use `IStringLocalizer` for dynamic string localization.
- Introduced the `L(string key, string fallback)` helper method for consistent localized string retrieval with fallback support.
- Implemented dynamic culture initialization in `Program.cs` via an API call to `/api/Localization/lang`.
- Localized error pages, grid columns, tabs, popups, validation messages, and tooltips.
- Dynamically generated privacy policy links based on the current culture.
- Improved user experience with fully localized loading states, error handling, and UI labels.

These changes enhance accessibility and usability, making the application more user-friendly for a global audience.

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

@@ -5,9 +5,17 @@ using DevExpress.Blazor.Reporting;
 using DevExpress.XtraReports.Web.Extensions;
 using DevExpress.DataAccess.Web;
 using DevExpress.XtraReports.Services;
+using System.Globalization;
 
 var builder = WebAssemblyHostBuilder.CreateDefault(args);
 
+var initialCulture = await ResolveInitialCultureAsync(builder.HostEnvironment.BaseAddress);
+var cultureInfo = new CultureInfo(initialCulture);
+CultureInfo.DefaultThreadCurrentCulture = cultureInfo;
+CultureInfo.DefaultThreadCurrentUICulture = cultureInfo;
+
+builder.Services.AddLocalization();
+
 // Named HttpClient for API calls (both for services and DevExpress components)
 builder.Services.AddHttpClient("EnvelopeGenerator.Server", client =>
 {
@@ -67,3 +75,23 @@ ReportStorageWebExtension.RegisterExtensionGlobal(new InMemoryReportStorageWebEx
 var host = builder.Build();
 await FontLoader.LoadFonts(host.Services.GetRequiredService<IHttpClientFactory>(), new List<string> { "opensans.ttf" });
 await host.RunAsync();
+
+static async Task<string> ResolveInitialCultureAsync(string baseAddress)
+{
+    try
+    {
+        using var http = new HttpClient { BaseAddress = new Uri(baseAddress) };
+        var response = await http.GetAsync("/api/Localization/lang");
+
+        if (!response.IsSuccessStatusCode)
+            return "de-DE";
+
+        var value = await response.Content.ReadAsStringAsync();
+        var culture = value.Trim().Trim('"');
+        return string.IsNullOrWhiteSpace(culture) ? "de-DE" : culture;
+    }
+    catch
+    {
+        return "de-DE";
+    }
+}