[license] client/simple: SPDX-License-Identifier: AGPL-3.0-or-later

SPDX short-form identifiers to communicate license information in a simple,
efficient, portable and machine-readable manner [1]

[1] https://spdx.dev/learn/handling-license-info/

client/simple/src/less/embedded.less

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
 iframe[src^="https://w.soundcloud.com"]
 {
   height: 120px;