Add prev/next navigation buttons to Monthly Check-in view

Adds chevron buttons to navigate between months, similar to Lose It Log view.
The next month button is disabled when already at the current month.

Fixes #20

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

PortfolioJournal/Views/Dashboard/MonthlyCheckInView.swift

@@ -4,7 +4,7 @@ struct MonthlyCheckInView: View {
     @Environment(\.openURL) private var openURL
     @EnvironmentObject var accountStore: AccountStore
     @StateObject private var viewModel = MonthlyCheckInViewModel()
-    let referenceDate: Date
+    @State private var referenceDate: Date
     let duplicatePrevious: Bool
 
     @State private var monthlyNote: String
@@ -18,7 +18,7 @@ struct MonthlyCheckInView: View {
     @State private var showAppStoreReviewAlert = false
 
     init(referenceDate: Date = Date(), duplicatePrevious: Bool = false) {
-        self.referenceDate = referenceDate
+        _referenceDate = State(initialValue: referenceDate)
         self.duplicatePrevious = duplicatePrevious
         _monthlyNote = State(initialValue: MonthlyCheckInStore.note(for: referenceDate))
         _starRating = State(initialValue: MonthlyCheckInStore.rating(for: referenceDate) ?? 0)
@@ -74,6 +74,21 @@ struct MonthlyCheckInView: View {
         return .appSecondary
     }
 
+    private var canGoToNextMonth: Bool {
+        guard let nextMonth = Calendar.current.date(byAdding: .month, value: 1, to: referenceDate) else { return false }
+        return nextMonth <= Date()
+    }
+
+    private func navigateMonth(offset: Int) {
+        guard let newDate = Calendar.current.date(byAdding: .month, value: offset, to: referenceDate) else { return }
+        referenceDate = newDate
+        monthlyNote = MonthlyCheckInStore.note(for: newDate)
+        starRating = MonthlyCheckInStore.rating(for: newDate) ?? 0
+        selectedMood = MonthlyCheckInStore.mood(for: newDate)
+        viewModel.selectedRange = DateRange.month(containing: newDate)
+        viewModel.refresh()
+    }
+
     private var canAddNewCheckIn: Bool {
         true
     }
@@ -167,8 +182,31 @@ struct MonthlyCheckInView: View {
 
     private var headerCard: some View {
         VStack(alignment: .leading, spacing: 8) {
-            Text(monthLabel)
+            HStack {
+                Button {
+                    navigateMonth(offset: -1)
+                } label: {
+                    Image(systemName: "chevron.left")
                         .font(.headline)
+                        .foregroundColor(.appPrimary)
+                }
+
+                Spacer()
+
+                Text(monthLabel)
+                    .font(.title3.weight(.bold))
+
+                Spacer()
+
+                Button {
+                    navigateMonth(offset: 1)
+                } label: {
+                    Image(systemName: "chevron.right")
+                        .font(.headline)
+                        .foregroundColor(canGoToNextMonth ? .appPrimary : .secondary.opacity(0.3))
+                }
+                .disabled(!canGoToNextMonth)
+            }
 
             if let date = lastCompletionDate {
                 Text(