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/tests/components/admin/users/UserActionMenu.test.tsx

@@ -510,8 +510,6 @@ describe('UserActionMenu', () => {
 
   describe('User Name Display', () => {
     it('displays full name when last name is provided', async () => {
-      const user = userEvent.setup();
-
       render(
         <UserActionMenu
           user={mockUser}
@@ -527,7 +525,6 @@ describe('UserActionMenu', () => {
     });
 
     it('displays first name only when last name is null', async () => {
-      const user = userEvent.setup();
       const userWithoutLastName = { ...mockUser, last_name: null };
 
       render(