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/en.ts

@@ -550,6 +550,9 @@ export const en: Translations = {
     validationInvalidCss: 'A static page contains invalid CSS.',
     validationDuplicateRoutes: 'Two or more pages share the same route.',
     validationInvalidWidgetConfig: 'A widget is missing a required field (id, type, version, or props).',
+    validationInvalidContactEmail: 'The company contact email is not a valid email address.',
+    validationInvalidSocialLinkUrl: 'One or more footer social links have an invalid URL.',
+    validationIncompletePaymentIcon: 'A footer payment icon is missing its image or alt text.',
     undo: 'Undo',
     redo: 'Redo',
     statusDraft: 'Draft',