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/Program.cs

@@ -33,6 +33,7 @@ builder.Services.AddScoped<SignatureService>();
 builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
+builder.Services.AddScoped<LocalizationService>();
 
 // UI Preferences & Theme
 builder.Services.AddScoped<UserPreferencesService>();