Rewrite old E2E tests to use fixture-based API mocking, eliminating need for real credentials

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cypress.config.js

@@ -10,7 +10,5 @@ module.exports = defineConfig({
     specPattern: "cypress/e2e/**/*.{cy,spec}.{js,jsx,ts,tsx}",
   },
   includeShadowDom: true,
-  env: {
-    // Removed hardcoded password — use cypress.env.json for real credentials
-  }
+  env: {}
 });