Expand Vitest coverage for shared frontend utils.

Cover Axios error message extraction, generic error fallbacks, ArrayBuffer views used by Web Crypto, and share URL builders.

frontend/src/utils/share/buildZkViewerPath.spec.ts

@@ -0,0 +1,11 @@
+import { describe, expect, it } from 'vitest';
+import { buildZkViewerPath } from '@/utils/share/buildZkViewerPath';
+
+describe('buildZkViewerPath', () => {
+    it('places upload id in the path and fragment key in the hash', () => {
+        const uploadId = 'abc-123';
+        const fragmentKey = 'dGVzdC1rZXk';
+
+        expect(buildZkViewerPath(uploadId, fragmentKey)).toBe(`/v/${uploadId}#${fragmentKey}`);
+    });
+});