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/Repositories/SnapshotRepository.swift

@@ -148,6 +148,40 @@ class SnapshotRepository: ObservableObject {
         save()
     }
 
+    // MARK: - Contribution Propagation
+
+    enum ContributionPropagation {
+        case forward   // snapshots after the pivot
+        case backward  // snapshots before the pivot
+        case all       // every other snapshot of the source
+    }
+
+    /// Applies `amount` as the contribution of other snapshots of the same source,
+    /// relative to `snapshot`'s date, according to `direction`. The pivot snapshot
+    /// itself is left untouched (it was already saved with its own value).
+    func propagateContribution(
+        _ amount: Decimal,
+        from snapshot: Snapshot,
+        direction: ContributionPropagation
+    ) {
+        guard let source = snapshot.source,
+              let set = source.snapshots as? Set<Snapshot> else { return }
+        let pivot = snapshot.date
+        let targets = set.filter { other in
+            guard other.objectID != snapshot.objectID else { return false }
+            switch direction {
+            case .forward: return other.date > pivot
+            case .backward: return other.date < pivot
+            case .all: return true
+            }
+        }
+        let value = NSDecimalNumber(decimal: amount)
+        for target in targets {
+            target.contribution = value
+        }
+        save()
+    }
+
     // MARK: - Delete
 
     func deleteSnapshot(_ snapshot: Snapshot) {