allow clients with no privateKey

see https://github.com/wg-easy/wg-easy/discussions/791

I disabled the download from the frontend.
If someone tries to manually call the API, the privateKey block is set to "REPLACE_ME" to indicate that this value must be set manually.

src/www/css/app.css

@@ -1334,6 +1334,11 @@ video {
   transition-duration: 150ms;
 }
 
+.disabled-link {
+  opacity: 0.25;
+  cursor: default;
+}
+
 .duration-200 {
   transition-duration: 200ms;
 }