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/LanguageControls.razor

@@ -1,14 +1,17 @@
 @using EnvelopeGenerator.Server.Client.Services
+@using EnvelopeGenerator.Application.Resources
+@using Microsoft.Extensions.Localization
 @rendermode InteractiveWebAssembly
 
 @inject LocalizationService LocalizationService
 @inject NavigationManager Navigation
+@inject IStringLocalizer<Resource> UiLocalizer
 
 <div class="sf-lang-combo">
     <button type="button"
             class="sf-lang-trigger"
             @onclick="ToggleMenu"
-            aria-label="Language"
+            aria-label="@L("Language", "Language")"
             aria-expanded="@_isOpen">
         <span class="fi @_selected.FlagClass"></span>
     </button>
@@ -17,11 +20,11 @@
     {
         <button type="button"
                 class="sf-lang-backdrop"
-                aria-label="Close language menu"
+                aria-label="@L("CloseLanguageMenu", "Close language menu")"
                 @onclick="CloseMenu">
         </button>
 
-        <div class="sf-lang-menu" role="listbox" aria-label="Languages">
+        <div class="sf-lang-menu" role="listbox" aria-label="@L("Languages", "Languages")">
             @foreach (var option in _options)
             {
                 <button type="button"
@@ -81,4 +84,10 @@
     }
 
     private sealed record LanguageOption(string Code, string FlagClass, string ShortName);
+
+    string L(string key, string fallback)
+    {
+        var value = UiLocalizer[key];
+        return value.ResourceNotFound ? fallback : value.Value;
+    }
 }