chore: enable Cerberus security by default and fix 31 skipped E2E tests

Phase 1 of skipped Playwright tests remediation:

Changed Cerberus default from disabled to enabled in backend code
Deprecated FEATURE_CERBERUS_ENABLED env var (no longer needed)
Added data-testid and a11y attributes to LanguageSelector component
Fixed keyboard navigation timing in account-settings and user-management tests
Simplified security dashboard toggle tests with waitForToast pattern
Test results: 668 passed, 11 failed, 67 skipped (reduced from 98)
Backend coverage: 87.0% (exceeds 85% threshold)

.docker/compose/docker-compose.e2e.yml

@@ -30,7 +30,7 @@ services:
       - CHARON_CADDY_CONFIG_DIR=/app/data/caddy
       - CHARON_CADDY_BINARY=caddy
       - CHARON_ACME_STAGING=true
-      - FEATURE_CERBERUS_ENABLED=false
+      # FEATURE_CERBERUS_ENABLED deprecated - Cerberus enabled by default
     volumes:
       # Use tmpfs for E2E test data - fresh on every run
       - e2e_data:/app/data