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/admin-organization-members.spec.ts

@@ -5,7 +5,7 @@
  */
 
 import { test, expect } from '@playwright/test';
-import { setupSuperuserMocks, loginViaUI } from './helpers/auth';
+import { setupSuperuserMocks } from './helpers/auth';
 
 test.describe('Admin Organization Members - Navigation from Organizations List', () => {
   test.beforeEach(async ({ page }) => {