Add Language Selector component to footer

Introduced a Language Selector component (`LanguageSelector.razor`) to enable users to switch between German, English, and French. The component includes a dropdown UI with flag icons and language labels, along with logic for toggling the dropdown and changing the app's culture.

Injected `IJSRuntime`, `NavigationManager`, and `CultureService` into the component to handle culture changes and navigation updates.

Updated `MainLayout.razor` to include the Language Selector in the footer. Refactored footer layout in `MainLayout.razor.css` to use flexbox for better alignment and spacing.

Added styles for the Language Selector in `app.css` to ensure a clean and responsive design. Integrated the `flag-icons` library in `index.html` to display flag icons for language representation.

EnvelopeGenerator.ReceiverUI/wwwroot/index.html

@@ -12,6 +12,7 @@
     <link href="css/bootstrap/bootstrap.min.css" rel="stylesheet" />
     <link href="EnvelopeGenerator.ReceiverUI.styles.css" rel="stylesheet" />
     <link href="css/app.css" rel="stylesheet" />
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/lipis/flag-icons@7.2.3/css/flag-icons.min.css" />
     <style type="text/css">
         .splash-screen {
             display: flex;