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/router.py

@@ -96,13 +96,12 @@ async def get_chat(
     current_user: User = Depends(get_current_user),
 ) -> ChatDetailRead:
     chat, members = await get_chat_for_user(db, chat_id=chat_id, user_id=current_user.id)
-    return ChatDetailRead(
-        id=chat.id,
-        type=chat.type,
-        title=chat.title,
-        pinned_message_id=chat.pinned_message_id,
-        created_at=chat.created_at,
-        members=members,
+    base = await serialize_chat_for_user(db, user_id=current_user.id, chat=chat)
+    return ChatDetailRead.model_validate(
+        {
+            **base.model_dump(),
+            "members": members,
+        }
     )