feat(usability): confirm saves and guard booking submissions against duplicates (U07)

src/Duempelkas.Infrastructure/Services/EntryService.cs

@@ -121,7 +121,7 @@ public class EntryService : IEntryService
         return new EntryDto(entry.Id, entry.AccountId, entry.DisplayId, entry.Type, entry.Date, entry.Title, entry.Amount, false, false, null, null, null);
     }
 
-    public async Task CreateTransferAsync(int sourceAccountId, int targetAccountId, DateTime date, string title, decimal amount)
+    public async Task<int> CreateTransferAsync(int sourceAccountId, int targetAccountId, DateTime date, string title, decimal amount)
     {
         BookingInput.Validate(date, title, amount);
         if (sourceAccountId == targetAccountId)
@@ -173,6 +173,7 @@ public class EntryService : IEntryService
         await db.SaveChangesAsync();
 
         await transaction.CommitAsync();
+        return sourceEntry.Id;
     }
 
     public async Task DeleteEntryAsync(int entryId)