Add multilingual support with language selector

Introduced a `LanguageControls` component for switching between
languages (`de-DE`, `en-US`, `fr-FR`) and integrated a new
`LocalizationService` to manage language preferences. Updated
`LocalizationController` to handle language persistence via
cookies and support multiple cultures. Enhanced the footer
layout to include the language selector alongside theme
controls.

Added `flag-icons` library for displaying country flags and
configured request localization in `Program.cs`. Styled the
language selector dropdown and improved sender page visuals
with updated CSS. Defaulted language handling to `de-DE` for
fallback scenarios. Cleaned up redundant code in
`LocalizationController`.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Layout/LanguageControls.razor

@@ -0,0 +1,84 @@
+@using EnvelopeGenerator.Server.Client.Services
+@rendermode InteractiveWebAssembly
+
+@inject LocalizationService LocalizationService
+@inject NavigationManager Navigation
+
+<div class="sf-lang-combo">
+    <button type="button"
+            class="sf-lang-trigger"
+            @onclick="ToggleMenu"
+            aria-label="Language"
+            aria-expanded="@_isOpen">
+        <span class="fi @_selected.FlagClass"></span>
+    </button>
+
+    @if (_isOpen)
+    {
+        <button type="button"
+                class="sf-lang-backdrop"
+                aria-label="Close language menu"
+                @onclick="CloseMenu">
+        </button>
+
+        <div class="sf-lang-menu" role="listbox" aria-label="Languages">
+            @foreach (var option in _options)
+            {
+                <button type="button"
+                        class="sf-lang-option @(_selected.Code == option.Code ? "sf-lang-option--active" : string.Empty)"
+                        role="option"
+                        aria-selected="@(_selected.Code == option.Code)"
+                        @onmousedown:preventDefault="true"
+                        @onclick="() => SelectLanguageAsync(option)">
+                    <span class="fi @option.FlagClass"></span>
+                    <span>@option.ShortName</span>
+                </button>
+            }
+        </div>
+    }
+</div>
+
+@code {
+    private readonly IReadOnlyList<LanguageOption> _options =
+    [
+        new("de-DE", "fi-de", "DE"),
+        new("en-US", "fi-us", "EN"),
+        new("fr-FR", "fi-fr", "FR")
+    ];
+
+    private LanguageOption _selected = new("de-DE", "fi-de", "DE");
+    private bool _isOpen;
+
+    protected override async Task OnInitializedAsync()
+    {
+        var currentLanguage = await LocalizationService.GetLanguageAsync();
+        _selected = _options.FirstOrDefault(x => x.Code.Equals(currentLanguage, StringComparison.OrdinalIgnoreCase))
+            ?? _options[0];
+    }
+
+    private void ToggleMenu()
+    {
+        _isOpen = !_isOpen;
+    }
+
+    private void CloseMenu()
+    {
+        _isOpen = false;
+    }
+
+    private async Task SelectLanguageAsync(LanguageOption option)
+    {
+        if (_selected.Code.Equals(option.Code, StringComparison.OrdinalIgnoreCase))
+        {
+            _isOpen = false;
+            return;
+        }
+
+        _selected = option;
+        _isOpen = false;
+        await LocalizationService.SetLanguageAsync(option.Code);
+        Navigation.NavigateTo(Navigation.Uri, forceLoad: true);
+    }
+
+    private sealed record LanguageOption(string Code, string FlagClass, string ShortName);
+}

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Layout/MainLayout.razor

@@ -21,7 +21,10 @@
             <a href="docs/privacy-policy.de-DE.html" target="_blank" rel="noopener">Datenschutz</a>
         </div>
 
-        <ThemeControls />
+        <div class="receiver-footer__right">
+            <LanguageControls />
+            <ThemeControls />
+        </div>
     </footer>
 </div>