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/Components/App.razor

@@ -6,6 +6,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <base href="/" />
     <link rel="stylesheet" href="css/bootstrap/bootstrap.min.css" />
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flag-icons@7.2.3/css/flag-icons.min.css" />
     <link rel="stylesheet" href="css/app.css" />
     <link rel="stylesheet" href="css/envelope-viewer.css" />
     <link rel="stylesheet" href="EnvelopeGenerator.Server.styles.css" />

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor

@@ -527,6 +527,7 @@
             </div>
         }
     </div>
+
 </div>
 
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