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)

frontend/src/components/LanguageSelector.tsx

@@ -21,6 +21,9 @@ export function LanguageSelector() {
     <div className="flex items-center gap-3">
       <Globe className="h-5 w-5 text-content-secondary" />
       <select
+        id="language-selector"
+        data-testid="language-selector"
+        aria-label="Language"
         value={language}
         onChange={handleChange}
         className="bg-surface-elevated border border-border rounded-md px-3 py-2 text-content-primary focus:outline-none focus:ring-2 focus:ring-primary focus:border-transparent transition-all"