refactor: normalize Author entity, eliminate magic strings, and improve exception handling per PR review

src/NexusReader.Web.New/Program.cs

@@ -458,7 +458,11 @@ app.MapGet("/identity/profile", async (ClaimsPrincipal user, UserManager<NexusUs
             {
                 Id = e.Id,
                 Title = e.Title,
-                Author = e.Author,
+                Author = new AuthorDto 
+                { 
+                    Id = e.Author.Id, 
+                    Name = e.Author.Name 
+                },
                 CoverUrl = e.CoverUrl,
                 Progress = 65,
                 LastChapter = "Chapter 4: Renaissance in Italy"

src/NexusReader.Web.New/Services/ServerIdentityService.cs

@@ -68,7 +68,11 @@ public class ServerIdentityService : IIdentityService
                 {
                     Id = e.Id,
                     Title = e.Title,
-                    Author = e.Author,
+                    Author = new AuthorDto 
+                    { 
+                        Id = e.Author.Id, 
+                        Name = e.Author.Name 
+                    },
                     CoverUrl = e.CoverUrl,
                     Progress = 65, // Hardcoded for now as per design requirements, will link to real segments later
                     LastChapter = "Chapter 4: Renaissance in Italy"