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/wwwroot/css/app.css

@@ -11,7 +11,7 @@ html, body {
 }
 
 article {
-    height: calc(100vh - 70px);
+    height: calc(100vh - 36px);
     display: flex;
     flex-direction: column;
     overflow: hidden;
@@ -68,7 +68,7 @@ article {
 }
 
 .dx-blazor-reporting-container {
-    height: calc(100vh - 130px) !important;
+    height: calc(100vh - 166px) !important;
     width: 100% !important;
 }
 
@@ -242,4 +242,38 @@ article {
     align-items: center;
     gap: 7px;
     flex-wrap: wrap;
+}
+
+/* ── Footer ──────────────────────────────────────────────────────────────── */
+.receiver-footer {
+    position: fixed;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    height: 36px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 10px;
+    padding: 0 20px;
+    background: linear-gradient(135deg, #2c3e50 0%, #3498db 100%);
+    color: rgba(255, 255, 255, 0.75);
+    font-size: 0.72rem;
+    z-index: 100;
+    flex-shrink: 0;
+}
+
+.receiver-footer a {
+    color: rgba(255, 255, 255, 0.90);
+    text-decoration: none;
+    transition: color 0.15s;
+}
+
+.receiver-footer a:hover {
+    color: #ffffff;
+    text-decoration: underline;
+}
+
+.receiver-footer__sep {
+    opacity: 0.4;
 }