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/scripts/convert-v8-to-istanbul.ts

@@ -83,7 +83,7 @@ async function convertV8ToIstanbul() {
     // Dynamic import to handle both scenarios (installed vs not installed)
     const module = await import('v8-to-istanbul');
     v8toIstanbul = module.default || module;
-  } catch (error) {
+  } catch {
     console.log('❌ v8-to-istanbul not installed\n');
     console.log('📦 Install it with:');
     console.log('   npm install -D v8-to-istanbul\n');