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

cypress/e2e/modals.cy.ts

@@ -272,6 +272,22 @@ describe("modals", () => {
 
   });
 
+  describe("add layer", () => {
+    beforeEach(() => {
+      when.setStyle("layer");
+      when.modal.open();
+    });
+
+    it("shows duplicate id error", () => {
+      when.setValue("add-layer.layer-id.input", "background");
+      when.click("add-layer");
+      then(get.elementByTestId("modal:add-layer")).shouldExist();
+      then(get.element(".maputnik-modal-error")).shouldContainText(
+        "Layer ID already exists"
+      );
+    });
+  });
+
   describe("sources", () => {
     it("toggle");
   });