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/css/app.css

@@ -365,4 +365,75 @@ article {
 
 .receiver-footer__sep {
     opacity: 0.4;
-}
+}
+
+/* ── Language Selector (Footer) ──────────────────────────────────────────── */
+.language-selector {
+    position: relative;
+    display: inline-block;
+}
+
+.language-selector__trigger {
+    display: flex;
+    align-items: center;
+    gap: 0.4rem;
+    padding: 0.25rem 0.5rem;
+    background: rgba(255, 255, 255, 0.1);
+    border: 1px solid rgba(255, 255, 255, 0.2);
+    border-radius: 4px;
+    color: rgba(255, 255, 255, 0.9);
+    cursor: pointer;
+    font-size: 1rem;
+    transition: all 0.2s ease;
+}
+
+.language-selector__trigger:hover {
+    background: rgba(255, 255, 255, 0.15);
+    border-color: rgba(255, 255, 255, 0.3);
+}
+
+.language-selector__arrow {
+    font-size: 0.6rem;
+    transition: transform 0.2s ease;
+    opacity: 0.7;
+}
+
+.language-selector__dropdown {
+    position: absolute;
+    bottom: calc(100% + 0.5rem);
+    right: 0;
+    background: white;
+    border: 1px solid #ddd;
+    border-radius: 6px;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+    min-width: 160px;
+    z-index: 1000;
+    overflow: hidden;
+}
+
+.language-selector__option {
+    display: flex;
+    align-items: center;
+    gap: 0.75rem;
+    width: 100%;
+    padding: 0.65rem 1rem;
+    background: white;
+    border: none;
+    color: #333;
+    cursor: pointer;
+    font-size: 0.9rem;
+    transition: background-color 0.2s ease;
+    text-align: left;
+}
+
+.language-selector__option:hover {
+    background-color: #f5f5f5;
+}
+
+.language-selector__option .fi {
+    font-size: 1.25rem;
+}
+
+.language-selector__option span:last-child {
+    font-weight: 500;
+}

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;