[PM-21741] Welcome email updates (#6479)

feat(PM-21741): implement MJML welcome email templates with feature flag support

- Add MJML templates for individual, family, and organization welcome emails
- Track *.hbs artifacts from MJML build
- Implement feature flag for gradual rollout of new email templates
- Update RegisterUserCommand and HandlebarsMailService to support new templates
- Add text versions and sanitization for all welcome emails
- Fetch organization data from database for welcome emails
- Add comprehensive test coverage for registration flow

Co-authored-by: Rui Tomé <108268980+r-tome@users.noreply.github.com>

src/Core/Platform/Mail/NoopMailService.cs

@@ -114,6 +114,20 @@ public class NoopMailService : IMailService
         return Task.FromResult(0);
     }
 
+    public Task SendIndividualUserWelcomeEmailAsync(User user)
+    {
+        return Task.FromResult(0);
+    }
+
+    public Task SendOrganizationUserWelcomeEmailAsync(User user, string organizationName)
+    {
+        return Task.FromResult(0);
+    }
+
+    public Task SendFreeOrgOrFamilyOrgUserWelcomeEmailAsync(User user, string familyOrganizationName)
+    {
+        return Task.FromResult(0);
+    }
     public Task SendVerifyDeleteEmailAsync(string email, Guid userId, string token)
     {
         return Task.FromResult(0);