Fix: make Receipt property nullable in ViewReceiptModel

Changed Receipt property from non-nullable (null!) to nullable (?) to properly
handle cases where receipt is not found. Fixes CS8601 null reference assignment
warning.

MoneyMap/Pages/ViewReceipt.cshtml.cs

@@ -23,7 +23,7 @@ namespace MoneyMap.Pages
             _parsers = parsers;
         }
 
-        public Receipt Receipt { get; set; } = null!;
+        public Receipt? Receipt { get; set; }
         public List<ReceiptLineItem> LineItems { get; set; } = new();
         public List<ReceiptParseLog> ParseLogs { get; set; } = new();
         public List<ParserOption> AvailableParsers { get; set; } = new();