fix(test): mock navigator.userAgent in API key storage tests

The ApiKeyStorage class uses navigator.userAgent to derive the
encryption key. Without mocking this value, tests fail in CI
environments where navigator may be undefined or different.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: msujaws

src/lib/storage/api-key-storage.test.ts

@@ -4,6 +4,7 @@ import { ApiKeyStorage } from './api-key-storage'
 describe('ApiKeyStorage', () => {
   let storage: ApiKeyStorage
   let localStorageMock: Record<string, string>
+  const mockUserAgent = 'Mozilla/5.0 (Test) TestBrowser/1.0'
 
   beforeEach(() => {
     // Mock localStorage
@@ -26,6 +27,13 @@ describe('ApiKeyStorage', () => {
       key: vi.fn(),
     }
 
+    // Mock navigator.userAgent for consistent encryption key derivation
+    Object.defineProperty(global, 'navigator', {
+      value: { userAgent: mockUserAgent },
+      writable: true,
+      configurable: true,
+    })
+
     storage = new ApiKeyStorage()
   })