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/Views/Dashboard/DashboardView.swift

@@ -416,9 +416,10 @@ struct MonthlyCheckInCard: View {
     @State private var showingStartOptions = false
     @State private var startDestinationActive = false
     @State private var shouldDuplicatePrevious = false
+    @State private var cachedCompletionDate: Date?
 
     private var effectiveLastCheckInDate: Date? {
-        MonthlyCheckInStore.latestCompletionDate() ?? lastUpdatedDate
+        cachedCompletionDate ?? lastUpdatedDate
     }
 
     private var checkInProgress: Double {
@@ -432,7 +433,8 @@ struct MonthlyCheckInCard: View {
 
     private var nextCheckInDate: Date? {
         guard let last = effectiveLastCheckInDate else { return nil }
-        return last.adding(months: 1)
+        let effective = MonthlyCheckInStore.effectiveMonth(for: last, relativeTo: last)
+        return effective.adding(months: 1).endOfMonth
     }
 
     private var isOverdue: Bool {
@@ -473,10 +475,10 @@ struct MonthlyCheckInCard: View {
 
     var body: some View {
         VStack(alignment: .leading, spacing: 12) {
-            Text(currentMonthLabel)
+            Text("Monthly Check-in")
                 .font(.headline)
 
-            Text("Keep a calm, deliberate rhythm. Update your sources and add a short note.")
+            Text(currentMonthLabel)
                 .font(.subheadline)
                 .foregroundColor(.secondary)
 
@@ -583,6 +585,14 @@ struct MonthlyCheckInCard: View {
             .padding(.horizontal, 24)
             .presentationDetents([.height(280)])
         }
+        .onAppear {
+            cachedCompletionDate = MonthlyCheckInStore.latestCompletionDate()
+        }
+        .onChange(of: startDestinationActive) { _, isActive in
+            if !isActive {
+                cachedCompletionDate = MonthlyCheckInStore.latestCompletionDate()
+            }
+        }
     }
 
     private var defaultReminderTime: Date {