Add duplicate layer id check (#1262)

## Summary

<img width="437" alt="Screenshot 2025-07-05 at 00 29 02"
src="https://github.com/user-attachments/assets/ebf8b024-a340-4eac-b470-29fd8f080c0a"
/>

- show an error if a layer with an existing id is added
- keep Add Layer modal open until the id is unique

## Testing
- `npm run lint`
- `npm run build`


------
https://chatgpt.com/codex/tasks/task_e_6868498a46188331b16e7b6e120930a7

CHANGELOG.md

@@ -19,6 +19,7 @@
 ### 🐞 Bug fixes
 
 - Fix incorrect handing of network error response (#944)
+- Show an error when adding a layer with a duplicate ID
 - _...Add new stuff here..._
 
 ## 2.1.1