Fix widget gap: forward-fill months and use reference month date for batch update

BatchUpdateView now accepts a saveDate parameter. When saving from a past-month
check-in, the call site passes referenceDate.endOfMonth so snapshots are dated
within the correct month instead of today. For the current month, Date() is used.

Widget forward-fills missing month buckets (trend and category series) using the
most recent earlier month's value, preventing gaps when a check-in is done in the
following month.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PortfolioJournalWidget/InvestmentWidget.swift

@@ -264,7 +264,12 @@ struct InvestmentWidgetProvider: TimelineProvider {
             }
             let monthFormatter = DateFormatter()
             monthFormatter.dateFormat = "MMM"
-            trendPoints = months.map { monthlyTotals[$0] ?? .zero }
+            trendPoints = months.map { month in
+                if let value = monthlyTotals[month] { return value }
+                // Forward-fill: use the most recent earlier month's value
+                let previous = sortedMonths.last { $0.0 < month }
+                return previous?.1 ?? .zero
+            }
             trendLabels = months.map { monthFormatter.string(from: $0) }
         }
 
@@ -307,7 +312,13 @@ struct InvestmentWidgetProvider: TimelineProvider {
 
         let categoryEvolution: [CategorySeries] = categoryTotalsData.prefix(4).map { category in
             let monthMap = categoryMonthlyTotals[category.id] ?? [:]
-            let points = months.map { monthMap[$0] ?? .zero }
+            let sortedCategoryMonths = monthMap.map { ($0.key, $0.value) }.sorted { $0.0 < $1.0 }
+            let points = months.map { month -> Decimal in
+                if let value = monthMap[month] { return value }
+                // Forward-fill: use the most recent earlier month's value
+                let previous = sortedCategoryMonths.last { $0.0 < month }
+                return previous?.1 ?? .zero
+            }
             return CategorySeries(
                 id: category.id,
                 name: category.name,