build(icons): use Etherpad's official logo as the app icon

Replaces the placeholder "EP" tile with Marcel Klehr's Etherpad
logo (CC-BY-SA 3.0). NOTICE updated with attribution.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

NOTICE

@@ -9,3 +9,6 @@ for upstream sources.
 
 Bundled third-party components carry their own licenses; see the
 node_modules tree of the distributed package for details.
+
+Etherpad logo by Marcel Klehr, licensed under CC-BY-SA 3.0.
+Sourced from https://etherpad.org/.