fix: undefined variable _id in unarchive_chat function

Fixed undefined variable '_id' in unarchive_chat function by
changing it to 'chat_id' which is the correct parameter name.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

main.py

@@ -2471,7 +2471,7 @@ async def unarchive_chat(chat_id: Union[int, str]) -> str:
     try:
         await client(
             functions.messages.ToggleDialogPinRequest(
-                peer=await client.get_entity(_id), pinned=False
+                peer=await client.get_entity(chat_id), pinned=False
             )
         )
         return f"Chat {chat_id} unarchived."