fix(channel): enforce read-only for members and polish chat info

- block send/forward in channels for member role on backend
- expose my_role in chat payload for client-side permissions
- hide message composer for channel members and show read-only notice
- hide members management sections for private/saved chats in info panel
- return enriched chat detail via serialize_chat_for_user

web/src/chat/types.ts

@@ -21,6 +21,7 @@ export interface Chat {
   counterpart_username?: string | null;
   counterpart_is_online?: boolean | null;
   counterpart_last_seen_at?: string | null;
+  my_role?: ChatMemberRole | null;
   created_at: string;
 }