Refactor ESLint configuration and update test rules for clarity and consistency

- Consolidated and modularized `eslint.config.mjs` with defined rules for source, test, E2E, and scripts.
- Improved test and E2E rules with relaxed settings for flexibility and enhanced mocking.
- Standardized variable naming and removed redundant imports in unit and E2E tests.
- Updated error handling and comments to align with modern TypeScript best practices (e.g., `@ts-expect-error`).

frontend/e2e/auth.setup.ts

@@ -11,9 +11,9 @@
  * - Savings: ~690s (~11 minutes) per test run
  */
 
-import { test as setup, expect } from '@playwright/test';
+import { test as setup } from '@playwright/test';
 import path from 'path';
-import { setupAuthenticatedMocks, setupSuperuserMocks, loginViaUI } from './helpers/auth';
+import { setupAuthenticatedMocks, setupSuperuserMocks } from './helpers/auth';
 
 // Use absolute paths to ensure correct file location
 const ADMIN_STORAGE_STATE = path.join(__dirname, '.auth', 'admin.json');