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/features/project-editor/sections/footer-section.component.ts

@@ -2,6 +2,7 @@ import { ChangeDetectionStrategy, Component, computed, inject } from '@angular/c
 import { FormsModule } from '@angular/forms';
 import { ProjectEditorFacade } from '../facade/project-editor.facade';
 import { TranslatePipe } from '../../../i18n/translate.pipe';
+import { TranslateService } from '../../../i18n/translate.service';
 import { InputComponent } from '../../../shared/ui/input/input.component';
 import { FormFieldComponent } from '../../../shared/ui/form-field/form-field.component';
 import { SectionCardComponent } from '../../../shared/ui/section-card/section-card.component';
@@ -28,7 +29,12 @@ import { isValidHttpUrl } from '../schema/validators/primitives';
 })
 export class ProjectEditorFooterSectionComponent {
   private readonly facade = inject(ProjectEditorFacade);
+  private readonly translate = inject(TranslateService);
   readonly bootstrap = this.facade.bootstrap;
+  readonly fieldError = (key: string): string | null => {
+    const messageKey = this.facade.fieldError(key);
+    return messageKey ? this.translate.t(messageKey) : null;
+  };
 
   readonly staticPagesValue = computed(() => (this.bootstrap()?.footer?.staticPageKeys ?? this.bootstrap()?.footer?.legalPageKeys ?? []).join(', '));
   readonly copyrightValue = computed(() => {