Add footer and privacy policy files to ReceiverUI

Introduced a new footer in `MainLayout.razor` with copyright
information, a link to "Digital Data GmbH," and a privacy
policy link. Added two new privacy policy files
(`privacy-policy.en-US.html` and `privacy-policy.fr-FR.html`)
to the project and configured them to always copy to the
output directory.

Updated `app.css` to adjust layout heights to accommodate
the footer. Added new styles for the footer, including a
gradient background, responsive design, and hover effects
for links.

EnvelopeGenerator.ReceiverUI/Shared/MainLayout.razor

@@ -7,6 +7,11 @@
             @Body
         </article>
     </main>
+    <footer class="receiver-footer">
+        <span>&copy; SignFlow 2023-2024 <a href="https://digitaldata.works" target="_blank" rel="noopener">Digital Data GmbH</a></span>
+        <span class="receiver-footer__sep">&#124;</span>
+        <a href="docs/privacy-policy.de-DE.html" target="_blank" rel="noopener">Datenschutz</a>
+    </footer>
 </div>
 
 @code {