Add type, channel, and receiver fields to chat model and filter messages by PUBLIC channel type in ChatService.

frontend/src/app/pages/altitude/chat/objects/chat-message.object.ts

@@ -4,8 +4,9 @@ export interface ChatMessage {
   uuid: string;
   timestamp: number;
   server: string;
-  //TODO [Stijn] [2026-07-18]: Handle channel types
-  //channel: Channel;
+  type: string;
+  channel: string;
+  receiver: string;
   messageJson: MiniMessageComponent;
   notBlocked: boolean;
 }