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/Dashboard/DashboardView.swift

@@ -11,6 +11,7 @@ struct DashboardView: View {
     @State private var showingCustomize = false
     @State private var sectionConfigs = DashboardLayoutStore.load()
     @AppStorage("calmModeEnabled") private var calmModeEnabled = true
+    @AppStorage("showForecast") private var showForecast = true
     @State private var pendingAlertDismissed = false
 
     init() {
@@ -165,7 +166,7 @@ struct DashboardView: View {
                     isPositive: calmModeEnabled
                         ? viewModel.latestPortfolioChange.absolute >= 0
                         : viewModel.isDayChangePositive,
-                    forecast: viewModel.portfolioForecast,
+                    forecast: showForecast ? viewModel.portfolioForecast : nil,
                     isPremium: iapService.isPremium,
                     onUnlockTap: {
                         viewModel.showingPaywall = true