Replace browser prompt()-based editing with proper Bootstrap 5 modal
dialogs for testimonials, services, FAQs, and leads. This provides
better UX with form validation, structured input fields, and i18n
support (ES/RU) instead of raw prompt dialogs.
- Add testimonialModal, serviceModal, faqModal, leadModal to admin.html
- Add show*/save* methods in admin.js for each entity type
- Wire leads.html 'Add lead' button to leadModal
- Add modal JS modules (FAQModal, LeadModal, ServiceModal)
- Add unit and e2e tests for modals and API client
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