feat: implement email template system with vendor overrides

Add comprehensive email template management for both admin and vendors:

Admin Features:
- Email templates management page at /admin/email-templates
- Edit platform templates with language support (en, fr, de, lb)
- Preview templates with sample variables
- Send test emails
- View email logs per template

Vendor Features:
- Email templates customization page at /vendor/{code}/email-templates
- Override platform templates with vendor-specific versions
- Preview and test customized templates
- Revert to platform defaults

Technical Changes:
- Migration for vendor_email_templates table
- VendorEmailTemplate model with override management
- Enhanced EmailService with language resolution chain
  (customer preferred -> vendor preferred -> platform default)
- Branding resolution (Wizamart default, removed for whitelabel)
- Platform-only template protection (billing templates)
- Admin and vendor API endpoints with full CRUD
- Updated seed script with billing and team templates

Files: 22 changed, ~3,900 lines added

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/partials/sidebar.html

@@ -137,6 +137,7 @@
     {{ section_header('Platform Settings', 'settingsSection') }}
     {% call section_content('settingsSection') %}
         {{ menu_item('settings', '/admin/settings', 'cog', 'General') }}
+        {{ menu_item('email-templates', '/admin/email-templates', 'mail', 'Email Templates') }}
         {# TODO: Implement profile and API keys pages #}
         {# {{ menu_item('profile', '/admin/profile', 'user-circle', 'Profile') }} #}
         {# {{ menu_item('api-keys', '/admin/api-keys', 'key', 'API Keys') }} #}