feat(theme): resolve database migration blocker and apply ThemePreference column

src/NexusReader.Data/Persistence/AppDbContext.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore;
 using NexusReader.Domain.Entities;
+using NexusReader.Domain.Enums;
 
 namespace NexusReader.Data.Persistence;
 
@@ -43,6 +44,10 @@ public class AppDbContext : IdentityDbContext<NexusUser>
             // Note: DefaultValue for int is 1 (which corresponds to 'Free' in our seed)
             entity.Property(u => u.SubscriptionPlanId)
                 .HasDefaultValue(1);
+
+            entity.Property(u => u.ThemePreference)
+                .HasConversion<int>()
+                .HasDefaultValue(ThemeMode.System);
         });
 
         modelBuilder.Entity<SubscriptionPlan>(entity =>