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/Layout/MainLayout.razor

@@ -1,8 +1,14 @@
 @using EnvelopeGenerator.Server.Client.Services
+@using EnvelopeGenerator.Application.Resources
+@using Microsoft.AspNetCore.Components.Routing
+@using Microsoft.Extensions.Localization
+@using System.Globalization
 @inherits LayoutComponentBase
 @implements IDisposable
 
 @inject ThemeService ThemeService
+@inject IStringLocalizer<Resource> UiLocalizer
+@inject NavigationManager Navigation
 
 <HeadContent>
     <link href="@ThemeService.ThemeCssHref" rel="stylesheet" />
@@ -18,7 +24,7 @@
         <div class="receiver-footer__left">
             <span>&copy; SignFlow 2023-2024 <a href="https://digitaldata.works" target="_blank" rel="noopener">Digital Data GmbH</a></span>
             <span class="receiver-footer__sep">&#124;</span>
-            <a href="docs/privacy-policy.de-DE.html" target="_blank" rel="noopener">Datenschutz</a>
+            <a href="@GetPrivacyPolicyPath()" target="_blank" rel="noopener">@L("PrivacyPolicy", "Datenschutz")</a>
         </div>
 
         <div class="receiver-footer__right">
@@ -35,6 +41,7 @@
 
     protected override async Task OnInitializedAsync()
     {
+        Navigation.LocationChanged += OnLocationChanged;
         await FontLoader.LoadFonts(HttpClientFactory, ["opensans.ttf"]);
         _themeChangeHandler = () => InvokeAsync(StateHasChanged);
         ThemeService.OnChange += _themeChangeHandler;
@@ -42,5 +49,31 @@
     }
 
     public void Dispose()
-        => ThemeService.OnChange -= _themeChangeHandler;
+    {
+        ThemeService.OnChange -= _themeChangeHandler;
+        Navigation.LocationChanged -= OnLocationChanged;
+    }
+
+    private void OnLocationChanged(object? sender, LocationChangedEventArgs e)
+        => _ = InvokeAsync(StateHasChanged);
+
+    private string GetPrivacyPolicyPath()
+    {
+        var current = CultureInfo.CurrentUICulture.Name;
+        var code = current switch
+        {
+            "de-DE" => "de-DE",
+            "en-US" => "en-US",
+            "fr-FR" => "fr-FR",
+            _ => "de-DE"
+        };
+
+        return $"docs/privacy-policy.{code}.html";
+    }
+
+    string L(string key, string fallback)
+    {
+        var value = UiLocalizer[key];
+        return value.ResourceNotFound ? fallback : value.Value;
+    }
 }