Remove redundant timeout parameters across E2E tests and add performance optimization documentation.

- Reduced E2E test execution time by removing unnecessary `{ timeout: 10000 }` overrides for assertions and element waits, relying on global `expect` configuration.
- Removed redundant `networkidle` wait states for faster form render validations.
- Documented comprehensive performance optimization strategies in `E2E_PERFORMANCE_OPTIMIZATION.md`.
- Added `E2E_COVERAGE_GUIDE.md` for integrating and merging E2E test coverage with unit test coverage.

frontend/e2e/settings-profile.spec.ts

@@ -27,7 +27,7 @@ test.describe('Profile Settings', () => {
 
     // Wait for form to be populated with user data (use label-based selectors)
     const firstNameInput = page.getByLabel(/first name/i);
-    await firstNameInput.waitFor({ state: 'visible', timeout: 10000 });
+    await firstNameInput.waitFor({ state: 'visible' });
 
     // Verify form fields are populated with mock user data
     await expect(firstNameInput).toHaveValue(MOCK_USER.first_name);
@@ -38,7 +38,7 @@ test.describe('Profile Settings', () => {
   test('should show email as read-only', async ({ page }) => {
     // Wait for form to load
     const emailInput = page.getByLabel(/email/i);
-    await emailInput.waitFor({ state: 'visible', timeout: 10000 });
+    await emailInput.waitFor({ state: 'visible' });
 
     // Verify email field is disabled or read-only
     const isDisabled = await emailInput.isDisabled();