refactor(front): extract shared utils and rewire pages

app/shared/constructeurUtils.ts

@@ -20,7 +20,7 @@ const toStringId = (value: unknown): string | null => {
   }
   if (trimmed.includes('/')) {
     const parts = trimmed.split('/').filter(Boolean);
-    return parts.length ? parts[parts.length - 1] : null;
+    return parts.length ? (parts[parts.length - 1] ?? null) : null;
   }
   return trimmed;
 };