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/helpers/coverage.ts

@@ -57,7 +57,7 @@ export async function startCoverage(
   try {
       await page.coverage.startJSCoverage({
       resetOnNavigation: options?.resetOnNavigation ?? false,
-      // @ts-ignore
+      // @ts-expect-error - includeRawScriptCoverage is not in official types but supported by Playwright
       includeRawScriptCoverage: options?.includeRawScriptCoverage ?? false,
     });
   } catch (error) {