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/Settings/SettingsView.swift

@@ -7,6 +7,7 @@ struct SettingsView: View {
     @StateObject private var viewModel: SettingsViewModel
     @EnvironmentObject private var adMobService: AdMobService
     @AppStorage("calmModeEnabled") private var calmModeEnabled = true
+    @AppStorage("showForecast") private var showForecast = true
     @AppStorage("cloudSyncEnabled") private var cloudSyncEnabled = false
     @AppStorage("faceIdEnabled") private var faceIdEnabled = false
     @AppStorage("pinEnabled") private var pinEnabled = false
@@ -568,6 +569,8 @@ struct SettingsView: View {
         Section {
             Toggle("Calm Mode", isOn: $calmModeEnabled)
 
+            Toggle("Show Forecast", isOn: $showForecast)
+
             NavigationLink {
                 AllocationTargetsView()
             } label: {
@@ -581,7 +584,7 @@ struct SettingsView: View {
         } header: {
             Text("Long-Term Focus")
         } footer: {
-            Text("Calm Mode hides short-term noise and advanced charts, keeping the app focused on monthly check-ins.")
+            Text("Calm Mode hides short-term noise and advanced charts. Show Forecast controls whether prediction data appears in portfolio and charts.")
         }
     }