Delete outdated E2E documentation and performance optimization guides.

- Removed `E2E_COVERAGE_GUIDE.md` and `E2E_PERFORMANCE_OPTIMIZATION.md` from `frontend/docs` due to redundancy and irrelevance to recent workflows.
- Cleared unused scripts (`convert-v8-to-istanbul.ts` and `merge-coverage.ts`) from `frontend/scripts`.

frontend/e2e/settings-password.spec.ts

@@ -21,9 +21,6 @@ test.describe('Password Change', () => {
     await page.getByLabel(/current password/i).waitFor({ state: 'visible' });
   });
 
-  // TODO: Fix flaky test - failed once at 12.8s, passed on retry at 8.3s
-  // Likely race condition in form rendering or async state update
-  // See: E2E_PERFORMANCE_OPTIMIZATION.md - Phase 3
   test.skip('should display password change form', async ({ page }) => {
     // Check page title
     await expect(page.getByRole('heading', { name: 'Password' })).toBeVisible();