Xcode 27: compila sin warnings y migra API deprecadas de SwiftUI

- Imports de CoreData que faltaban (MemberImportVisibility) en AccountsView,
  ImportDataView, ChartsContainerView y AddSourceView.
- Aislamiento MainActor: IsolatedDefaultValues en el target de la app,
  LinearGradient @MainActor, WatchSyncMessage nonisolated, closures de
  ReviewPromptService/MonthlyCheckInStore/AppIntents.
- Warnings de valores sin usar (SnapshotGapDetector, DashboardLayoutStore,
  OnboardingView, GoalEditorView, CoreDataStack, SettingsViewModel).
- foregroundColor→foregroundStyle, cornerRadius→clipShape,
  navigationBarLeading/Trailing→topBarLeading/Trailing,
  Task.sleep(nanoseconds:)→sleep(for:), NavigationLink(isActive:)→
  navigationDestination(isPresented:).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F1u4K16xy7eQVtgsYNZ9Vn

PortfolioJournal/Utilities/SnapshotGapDetector.swift

@@ -31,7 +31,6 @@ enum SnapshotGapDetector {
     /// Only months strictly between two known snapshots count — a source's
     /// trailing "not updated in a while" is handled elsewhere (pending updates).
     static func detectGaps(sources: [InvestmentSource], snapshots: [Snapshot]) -> [Gap] {
-        let calendar = Calendar.current
         let snapshotsBySource = Dictionary(grouping: snapshots) { $0.source?.id }
         var gaps: [Gap] = []