Add setting to toggle forecast visibility in charts and dashboard

Adds a "Show Forecast" toggle in Settings > Long-Term Focus section.
When disabled, hides the forecast from the total portfolio card and
removes the prediction chart type from the charts tab.

Fixes #26

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

PortfolioJournal/Views/Charts/ChartsContainerView.swift

@@ -6,6 +6,7 @@ struct ChartsContainerView: View {
     @StateObject private var viewModel: ChartsViewModel
     @StateObject private var goalsViewModel = GoalsViewModel()
     @AppStorage("calmModeEnabled") private var calmModeEnabled = true
+    @AppStorage("showForecast") private var showForecast = true
 
     init(iapService: IAPService) {
         _viewModel = StateObject(wrappedValue: ChartsViewModel(iapService: iapService))
@@ -92,7 +93,7 @@ struct ChartsContainerView: View {
     private var chartTypeSelector: some View {
         ScrollView(.horizontal, showsIndicators: false) {
             HStack(spacing: 12) {
-                ForEach(viewModel.availableChartTypes(calmModeEnabled: calmModeEnabled)) { chartType in
+                ForEach(viewModel.availableChartTypes(calmModeEnabled: calmModeEnabled).filter { showForecast || $0 != .prediction }) { chartType in
                     ChartTypeButton(
                         chartType: chartType,
                         isSelected: viewModel.selectedChartType == chartType,