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/support/intercept.js

@@ -130,6 +130,11 @@ export function setupApiIntercepts() {
     fixture: 'api/filter-params.json',
   }).as('getFilterCheckParams');
 
+  // Compare dashboard
+  cy.intercept('GET', /\/api\/compare\?datasets=/, {
+    fixture: 'api/compare.json',
+  }).as('getCompare');
+
   // Dependents (for delete confirmation)
   cy.intercept('GET', '/api/logs/*/dependents', {
     statusCode: 200,