feat(project-editor): footer validation rules (contact email, social link URLs, payment icons)

Real gaps, not fabricated: isValidEmail existed in primitives.ts but was
never called anywhere; social-link URL check only lived as a per-row
template hint (never blocked publish or set the nav badge); payment icons
with only src or only alt set were silently accepted.

- invalid-contact-email: company.contacts.email must be a valid email (error)
- invalid-social-link-url: footer.socialLinks entries need a valid http(s) URL (warning)
- incomplete-payment-icon: a payment icon needs both src and alt, or neither (warning)

Wired into footer-section via the existing fieldError() pattern. Header has
no equivalent gap today (every header field is a bool/enum, always valid by
construction) so nothing was added there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translations.ts

@@ -548,6 +548,9 @@ export interface Translations {
     validationInvalidCss: string;
     validationDuplicateRoutes: string;
     validationInvalidWidgetConfig: string;
+    validationInvalidContactEmail: string;
+    validationInvalidSocialLinkUrl: string;
+    validationIncompletePaymentIcon: string;
     undo: string;
     redo: string;
     statusDraft: string;