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

src/locales/de/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "Einklappen",
   "Expand": "Ausklappen",
   "Add Layer": "Ebene hinzufügen",
+  "Layer ID already exists": "Layer-ID existiert bereits",
   "Search": "Suche",
   "Zoom:": "Zoom:",
   "Close popup": "Popup schließen",