Normalize all DateTime values to UTC across the codebase

Npgsql requires DateTimeKind.Utc for timestamp with time zone
columns. This fixes errors in transaction create/edit, budgets,
reports, loans, scheduled transactions, and file imports.

Changes:
- TransactionService: SpecifyKind on request.Date for create,
  update, and transfer
- BudgetService: UTC kind on all new DateTime(year, month, 1)
- ReportService: UTC kind on endOfMonth; rewrite spending-by-
  category GroupBy to avoid untranslatable navigation properties
- LoanService: UTC kind on payoff scenario date construction
- ScheduledTransactionService: UTC kind on SemiMonthly date
- ImportService: SpecifyKind on parsed.Date when persisting
- OFX/CSV/QIF parsers: AssumeUniversal + AdjustToUniversal on
  all date parsing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Plugins.QIF/QIFParserPlugin.cs

@@ -47,7 +47,7 @@ public class QIFParserPlugin : IFileParser
                     case 'D':
                         current ??= new ParsedTransaction();
                         if (DateTime.TryParse(value, System.Globalization.CultureInfo.InvariantCulture,
-                            System.Globalization.DateTimeStyles.None, out var date))
+                            System.Globalization.DateTimeStyles.AssumeUniversal | System.Globalization.DateTimeStyles.AdjustToUniversal, out var date))
                             current.Date = date;
                         else if (TryParseQIFDate(value, out var qifDate))
                             current.Date = qifDate;
@@ -105,6 +105,6 @@ public class QIFParserPlugin : IFileParser
 
         return DateTime.TryParseExact(cleaned, formats,
             System.Globalization.CultureInfo.InvariantCulture,
-            System.Globalization.DateTimeStyles.None, out date);
+            System.Globalization.DateTimeStyles.AssumeUniversal | System.Globalization.DateTimeStyles.AdjustToUniversal, out date);
     }
 }