fix: skip backend fixture comparison when the frontend builds alone

The frontend image build only sees ApiCore-Frontend, so reading
../ApiCore-Backend/test/fixtures/placeholder-keys.json failed with ENOENT
in CI. Same pattern as the labels and bulk specs: compare in the monorepo
checkout, skip inside the image build.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/lib/label-templates/placeholder-keys.spec.ts

@@ -1,19 +1,25 @@
-import { readFileSync } from "node:fs";
+import { existsSync, readFileSync } from "node:fs";
 import path from "node:path";
 import { describe, expect, it } from "vitest";
 import { PLACEHOLDER_KEYS, PLACEHOLDER_LABELS } from "./placeholder-keys";
 
-describe("placeholder keys", () => {
-  it("the frontend list equals the backend fixture ApiCore-Backend/test/fixtures/placeholder-keys.json", () => {
 // The monorepo sits next to the frontend; vitest runs from ApiCore-Frontend.
+// The frontend image build only sees ApiCore-Frontend, so the comparison runs
+// in the monorepo checkout (local gates) and is skipped inside that build.
 const fixture = path.resolve(
   process.cwd(),
   "../ApiCore-Backend/test/fixtures/placeholder-keys.json",
 );
+
+describe("placeholder keys", () => {
+  it.skipIf(!existsSync(fixture))(
+    "the frontend list equals the backend fixture ApiCore-Backend/test/fixtures/placeholder-keys.json",
+    () => {
       const backendKeys: unknown = JSON.parse(readFileSync(fixture, "utf8"));
 
       expect([...PLACEHOLDER_KEYS]).toEqual(backendKeys);
-  });
+    },
+  );
 
   it("has a German label for every key", () => {
     for (const key of PLACEHOLDER_KEYS) {