1.4.1 (build 42): contribución por snapshot, fixes Period vs Period y navegación iPad

- 151: contribución editable por snapshot en cualquier modo (quitado gate detailed)
  con diálogo de propagación al editar: solo este / adelante / atrás / todos
  (SnapshotRepository.propagateContribution, SnapshotFormViewModel.contributionChanged)
- 148: Period vs Period mostraba mal el último mes del period B — la agrupación usaba
  chartMonth(for:) que aplicaba el grace-period del check-in a snapshots históricos;
  ahora agrupa por mes calendario crudo
- 152: iPad Sources no saltaba entre fuentes — añadido .id() al SourceDetailView para
  recrear el StateObject al cambiar de selección
- 146/147: Year vs Year con forecast del año en curso (asterisco de estimado) y KPIs
  arriba / detalle debajo
- 145: card de contribución mensual en SourceDetailView
- Nuevas claves snapshot_contribution_propagate_* en los 7 idiomas

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015qUZrBusG82T37R7PeokqJ

PortfolioJournal/ViewModels/SnapshotFormViewModel.swift

@@ -30,6 +30,11 @@ class SnapshotFormViewModel: ObservableObject {
     let mode: Mode
     let source: InvestmentSource
 
+    /// Contribution stored on the snapshot when editing started (nil in add mode or
+    /// when the snapshot had no contribution). Used to detect whether the user changed
+    /// the contribution and should be offered to propagate it to other snapshots.
+    private(set) var originalContribution: Decimal?
+
     // MARK: - Dependencies
 
     private var cancellables = Set<AnyCancellable>()
@@ -63,6 +68,7 @@ class SnapshotFormViewModel: ObservableObject {
             if let contribution = snapshot.contribution {
                 includeContribution = true
                 contributionString = formatDecimalForInput(contribution.decimalValue)
+                originalContribution = contribution.decimalValue
             }
             notes = snapshot.notes ?? ""
         }
@@ -128,6 +134,12 @@ class SnapshotFormViewModel: ObservableObject {
         return parseDecimal(trimmed)
     }
 
+    /// True when the current contribution differs from the value the snapshot had when
+    /// editing started. Drives the "propagate to other snapshots" prompt.
+    var contributionChanged: Bool {
+        contribution != originalContribution
+    }
+
     var formattedValue: String {
         guard let value = value else { return CurrencyFormatter.format(Decimal.zero) }
         return value.currencyString