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",

src/locales/fr/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "Réduire",
   "Expand": "Développer",
   "Add Layer": "Ajouter un calque",
+  "Layer ID already exists": "L'identifiant du calque existe déjà",
   "Search": "Recherche",
   "Zoom:": "Zoom :",
   "Close popup": "Fermer la fenêtre",

src/locales/he/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "הקטנה",
   "Expand": "הגדלה",
   "Add Layer": "הוספת שכבה",
+  "Layer ID already exists": "מזהה השכבה כבר קיים",
   "Search": "חיפוש",
   "Zoom:": "זום:",
   "Close popup": "סגירת החלון",

src/locales/it/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "Coprimi",
   "Expand": "Espandi",
   "Add Layer": "Aggiungi Livello",
+  "Layer ID already exists": "L'ID del layer esiste già",
   "Search": "Cerca",
   "Zoom:": "Zoom:",
   "Close popup": "Chiudi popup",

src/locales/ja/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "畳む",
   "Expand": "展開",
   "Add Layer": "レイヤー追加",
+  "Layer ID already exists": "レイヤーIDは既に存在します",
   "Search": "検索",
   "Zoom:": "ズーム:",
   "Close popup": "ポップアップを閉じる",

src/locales/zh/translation.json

@@ -72,6 +72,7 @@
   "Collapse": "折叠",
   "Expand": "展开",
   "Add Layer": "添加图层",
+  "Layer ID already exists": "图层ID已存在",
   "Search": "搜索",
   "Zoom:": "缩放:",
   "Close popup": "关闭弹出窗口",