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>

PortfolioJournal/Views/Dashboard/MonthlyCheckInView.swift

@@ -154,7 +154,8 @@ struct MonthlyCheckInView: View {
         .sheet(isPresented: $showBatchUpdate) {
             viewModel.refresh()
         } content: {
-            BatchUpdateView(sources: viewModel.sources)
+            let batchSaveDate = referenceDate.isSameMonth(as: Date()) ? Date() : referenceDate.endOfMonth
+            BatchUpdateView(sources: viewModel.sources, saveDate: batchSaveDate)
         }
         .onChange(of: starRating) { _, newValue in
             MonthlyCheckInStore.setRating(newValue == 0 ? nil : newValue, for: referenceDate)
@@ -945,10 +946,16 @@ struct AchievementMilestoneBar: View {
 struct BatchUpdateView: View {
     @Environment(\.dismiss) private var dismiss
     let sources: [InvestmentSource]
+    let saveDate: Date
     @State private var values: [UUID: String] = [:]
     @State private var contributions: [UUID: String] = [:]
     @State private var savedCount = 0
 
+    init(sources: [InvestmentSource], saveDate: Date = Date()) {
+        self.sources = sources
+        self.saveDate = saveDate
+    }
+
     var body: some View {
         NavigationStack {
             List {
@@ -1080,7 +1087,7 @@ struct BatchUpdateView: View {
 
             repository.createSnapshot(
                 for: source,
-                date: Date(),
+                date: saveDate,
                 value: parsed,
                 contribution: parsedContribution
             )