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/Services/LocalizationService.cs

@@ -0,0 +1,30 @@
+using System.Net;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+public class LocalizationService(IHttpClientFactory httpClientFactory)
+{
+    private HttpClient CreateDefaultClient() => httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+
+    public async Task<string> GetLanguageAsync(CancellationToken cancel = default)
+    {
+        using var http = CreateDefaultClient();
+        var response = await http.GetAsync("/api/Localization/lang", cancel);
+
+        if (response.StatusCode == HttpStatusCode.NotFound)
+            return "de-DE";
+
+        if (!response.IsSuccessStatusCode)
+            return "de-DE";
+
+        var language = await response.Content.ReadAsStringAsync(cancel);
+        return string.IsNullOrWhiteSpace(language) ? "de-DE" : language.Trim('"');
+    }
+
+    public async Task SetLanguageAsync(string language, CancellationToken cancel = default)
+    {
+        using var http = CreateDefaultClient();
+        var response = await http.PostAsync($"/api/Localization/lang?language={Uri.EscapeDataString(language)}", null, cancel);
+        response.EnsureSuccessStatusCode();
+    }
+}