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/Services/SampleDataService.swift

@@ -19,7 +19,6 @@ class SampleDataService {
 
         let snapshotRepository = SnapshotRepository(context: context)
         let goalRepository = GoalRepository(context: context)
-        let transactionRepository = TransactionRepository(context: context)
 
         let categories = fetchCategories(in: context)
         guard let fallbackCategory = categories.first else { return }
@@ -70,16 +69,6 @@ class SampleDataService {
 
         seedMonthlyNotes()
 
-        transactionRepository.createTransaction(
-            source: stocks,
-            type: .buy,
-            date: Calendar.current.date(byAdding: .month, value: -3, to: Date()) ?? Date(),
-            shares: 10,
-            price: 400,
-            amount: nil,
-            notes: "Sample buy"
-        )
-
         _ = goalRepository.createGoal(
             name: "1M Goal",
             targetAmount: 1_000_000,