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/Shared/LanguageSelector.razor

@@ -0,0 +1,62 @@
+@using System.Globalization
+@using EnvelopeGenerator.ReceiverUI.Services
+@inject IJSRuntime JSRuntime
+@inject NavigationManager Navigation
+@inject CultureService CultureService
+
+<div class="language-selector">
+    <button class="language-selector__trigger" @onclick="ToggleDropdown" aria-label="Select Language">
+        <span class="fi fi-@GetFlagCode(CurrentCulture)"></span>
+        <span class="language-selector__arrow">?</span>
+    </button>
+    
+    @if (isOpen)
+    {
+        <div class="language-selector__dropdown">
+            <button class="language-selector__option" @onclick="@(() => ChangeLanguageAsync("de-DE"))">
+                <span class="fi fi-de"></span>
+                <span>Deutsch</span>
+            </button>
+            <button class="language-selector__option" @onclick="@(() => ChangeLanguageAsync("en-US"))">
+                <span class="fi fi-us"></span>
+                <span>English</span>
+            </button>
+            <button class="language-selector__option" @onclick="@(() => ChangeLanguageAsync("fr-FR"))">
+                <span class="fi fi-fr"></span>
+                <span>Français</span>
+            </button>
+        </div>
+    }
+</div>
+
+@code {
+    private bool isOpen = false;
+    private string CurrentCulture => CultureInfo.CurrentCulture.Name;
+
+    private void ToggleDropdown()
+    {
+        isOpen = !isOpen;
+    }
+
+    private async Task ChangeLanguageAsync(string culture)
+    {
+        if (CultureInfo.CurrentCulture.Name != culture)
+        {
+            await CultureService.SetCultureAsync(culture);
+            Navigation.NavigateTo(Navigation.Uri, forceLoad: true);
+        }
+        
+        isOpen = false;
+    }
+
+    private string GetFlagCode(string culture)
+    {
+        return culture switch
+        {
+            "de-DE" => "de",
+            "en-US" => "us",
+            "fr-FR" => "fr",
+            _ => "de"
+        };
+    }
+}