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/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>