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

app/chats/schemas.py

@@ -26,6 +26,7 @@ class ChatRead(BaseModel):
     counterpart_username: str | None = None
     counterpart_is_online: bool | None = None
     counterpart_last_seen_at: datetime | None = None
+    my_role: ChatMemberRole | None = None
     created_at: datetime