1.5.0: aviso de huecos + resumen mensual compartible + import con vista previa

Feature 1 — Aviso de huecos: SnapshotGapDetector detecta huecos internos por
source (meses sin datos entre dos snapshots). Banner desechable en Dashboard +
DataGapsSheet para rellenar (abre AddSnapshotView con el mes que falta). Complementa
la interpolación de gráficas: la línea se ve suave pero el usuario sabe que faltan
datos reales.

Feature 2 — Resumen mensual compartible: MonthlySummaryShareView (tarjeta con marca:
valor, variación desde el check-in, racha, mood/rating) renderizada con ImageRenderer
y compartida vía ShareService. Botón en la fila "check-in done" del Dashboard.

Feature 3 — Import con vista previa: ImportPreview + previewImportAsync (dry-run
READ-ONLY, no muta nada) cuenta qué se creará/actualizará usando los mismos checks
de existencia que applyImport. ImportDataView muestra la confirmación antes de
importar. El path real de import (applyImport/importDataAsync) NO se toca.

Localización en los 7 idiomas.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L2p3gUZRNWW388rWFRjiU7

PortfolioJournal/ViewModels/DashboardViewModel.swift

@@ -25,6 +25,13 @@ class DashboardViewModel: ObservableObject {
     @Published var evolutionData: [(date: Date, value: Decimal)] = []
     @Published var updateStreak: Int = 0
 
+    // MARK: - Data Gaps
+
+    /// Internal gaps in per-source snapshot history (missing months between two
+    /// known snapshots). The chart smooths these over; this exposes them so the
+    /// user knows real data is missing and can fill it. See SnapshotGapDetector.
+    @Published var dataGaps: [SnapshotGapDetector.Gap] = []
+
     // MARK: - Portfolio Forecast
 
     @Published var portfolioForecast: PortfolioForecast?
@@ -209,6 +216,9 @@ class DashboardViewModel: ObservableObject {
         // Compute update streak
         updateStreak = computeStreak(from: evolutionData)
 
+        // Detect internal data gaps (missing months between known snapshots)
+        dataGaps = SnapshotGapDetector.detectGaps(sources: sources, snapshots: allSnapshots)
+
         // Log screen view
         FirebaseService.shared.logScreenView(screenName: "Dashboard")
     }
@@ -685,6 +695,42 @@ class DashboardViewModel: ObservableObject {
         sourcesNeedingUpdate.count
     }
 
+    /// Total number of missing months across all detected internal gaps.
+    var totalMissingMonths: Int {
+        SnapshotGapDetector.totalMissingMonths(dataGaps)
+    }
+
+    /// Resolves an InvestmentSource for a detected gap so the fill flow can open
+    /// the add-snapshot sheet for it.
+    func source(withId id: UUID) -> InvestmentSource? {
+        sourceRepository.sources.first { $0.id == id }
+    }
+
+    /// Builds the data for the shareable monthly summary card from already-computed
+    /// dashboard state. Mood/rating come from the latest completed check-in entry.
+    func monthlySummaryShareData() -> ShareService.MonthlySummaryShareData {
+        let monthLabel: String = {
+            let formatter = DateFormatter()
+            formatter.dateFormat = "MMMM yyyy"
+            let date = MonthlyCheckInStore.latestCompletionDate() ?? Date()
+            return formatter.string(from: date)
+        }()
+
+        let latestEntry = MonthlyCheckInStore.latestCompletionDate()
+            .flatMap { MonthlyCheckInStore.entry(for: $0) }
+
+        return ShareService.MonthlySummaryShareData(
+            monthLabel: monthLabel,
+            totalValue: portfolioSummary.formattedTotalValue,
+            changeAmount: latestPortfolioChange.formattedAbsolute,
+            changePercentage: latestPortfolioChange.formattedPercentage,
+            isPositive: latestPortfolioChange.absolute >= 0,
+            streak: updateStreak,
+            mood: latestEntry?.mood,
+            rating: latestEntry?.rating
+        )
+    }
+
     var insights: [PortfolioInsight] {
         var result: [PortfolioInsight] = []
         guard portfolioSummary.totalValue > 0 else { return result }