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/helpers/auth.ts

@@ -105,7 +105,7 @@ export async function loginViaUI(page: Page, email = 'test@example.com', passwor
 
   // Submit and wait for navigation to home
   await Promise.all([
-    page.waitForURL('/', { timeout: 10000 }),
+    page.waitForURL('/'),
     page.locator('button[type="submit"]').click(),
   ]);