Remove Add Transaction feature and clean all related code

Deleted files:
- AddTransactionView.swift
- TransactionRepository.swift
- Transaction+CoreDataClass.swift

Cleaned files:
- SourceDetailViewModel: removed transactions published property,
  showingAddTransaction flag, transactionRepository dependency,
  addTransaction() and deleteTransaction() methods
- SourceDetailView: removed transactionsSection, TransactionRow struct,
  Add Transaction button from quickActions, sheet presentation
- InvestmentSource+CoreDataClass: removed transactions NSManaged property,
  transactionsArray, totalInvested, totalDividends, totalFees computed
  properties, and all transaction Core Data accessors
- SettingsViewModel: removed "Transaction" from resetAllData entity list
- SampleDataService: removed transactionRepository and sample transaction

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

PortfolioJournal/Models/MonthlySummary.swift

@@ -15,6 +15,12 @@ struct MonthlySummary {
         return NSDecimalNumber(decimal: netPerformance / base).doubleValue * 100
     }
 
+    var formattedMonthYear: String {
+        let formatter = DateFormatter()
+        formatter.dateFormat = "MMMM yyyy"
+        return formatter.string(from: startDate)
+    }
+
     var formattedStartingValue: String {
         CurrencyFormatter.format(startingValue, style: .currency, maximumFractionDigits: 0)
     }