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/service.py

@@ -18,6 +18,10 @@ from app.users.repository import get_user_by_id
 
 async def serialize_chat_for_user(db: AsyncSession, *, user_id: int, chat: Chat) -> ChatRead:
     display_title = chat.title
+    my_role = None
+    membership = await repository.get_chat_member(db, chat_id=chat.id, user_id=user_id)
+    if membership:
+        my_role = membership.role
     members_count: int | None = None
     online_count: int | None = None
     subscribers_count: int | None = None
@@ -70,6 +74,7 @@ async def serialize_chat_for_user(db: AsyncSession, *, user_id: int, chat: Chat)
             "counterpart_username": counterpart_username,
             "counterpart_is_online": counterpart_is_online,
             "counterpart_last_seen_at": counterpart_last_seen_at,
+            "my_role": my_role,
             "created_at": chat.created_at,
         }
     )