Add monthly highlights: best and worst performing sources

Shows a highlights card with the best and worst performing sources by
percentage change, displayed between the summary and reflection cards.

Fixes #22

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

PortfolioJournal/Views/Dashboard/MonthlyCheckInView.swift

@@ -98,6 +98,7 @@ struct MonthlyCheckInView: View {
             VStack(spacing: 20) {
                 headerCard
                 summaryCard
+                monthlyHighlightsCard
                 reflectionCard
                 sourcesCard
                 notesCard
@@ -418,6 +419,73 @@ struct MonthlyCheckInView: View {
         .shadow(color: .black.opacity(0.05), radius: 8, y: 2)
     }
 
+    private var sourcePerformances: [(name: String, diff: Decimal, percentage: Double)] {
+        viewModel.sources.compactMap { source -> (name: String, diff: Decimal, percentage: Double)? in
+            let snapshots = source.sortedSnapshotsByDateAscending
+            guard snapshots.count >= 2 else { return nil }
+            let latest = snapshots[snapshots.count - 1]
+            let previous = snapshots[snapshots.count - 2]
+            let diff = latest.decimalValue - previous.decimalValue
+            let pct = previous.decimalValue > 0
+                ? NSDecimalNumber(decimal: diff / previous.decimalValue * 100).doubleValue
+                : 0
+            return (name: source.name, diff: diff, percentage: pct)
+        }
+    }
+
+    @ViewBuilder
+    private var monthlyHighlightsCard: some View {
+        let perfs = sourcePerformances
+        if perfs.count >= 2 {
+            let best = perfs.max(by: { $0.percentage < $1.percentage })
+            let worst = perfs.min(by: { $0.percentage < $1.percentage })
+            VStack(alignment: .leading, spacing: 12) {
+                Text("Monthly Highlights")
+                    .font(.headline)
+
+                if let best {
+                    HStack {
+                        Image(systemName: "arrow.up.circle.fill")
+                            .foregroundColor(.positiveGreen)
+                        VStack(alignment: .leading, spacing: 2) {
+                            Text("Best Performer")
+                                .font(.caption)
+                                .foregroundColor(.secondary)
+                            Text("\(best.name)")
+                                .font(.subheadline.weight(.semibold))
+                        }
+                        Spacer()
+                        Text(String(format: "%+.1f%%", best.percentage))
+                            .font(.subheadline.weight(.bold))
+                            .foregroundColor(.positiveGreen)
+                    }
+                }
+
+                if let worst, worst.name != best?.name {
+                    HStack {
+                        Image(systemName: "arrow.down.circle.fill")
+                            .foregroundColor(.negativeRed)
+                        VStack(alignment: .leading, spacing: 2) {
+                            Text("Worst Performer")
+                                .font(.caption)
+                                .foregroundColor(.secondary)
+                            Text("\(worst.name)")
+                                .font(.subheadline.weight(.semibold))
+                        }
+                        Spacer()
+                        Text(String(format: "%+.1f%%", worst.percentage))
+                            .font(.subheadline.weight(.bold))
+                            .foregroundColor(.negativeRed)
+                    }
+                }
+            }
+            .padding()
+            .background(Color(.systemBackground))
+            .cornerRadius(AppConstants.UI.cornerRadius)
+            .shadow(color: .black.opacity(0.05), radius: 8, y: 2)
+        }
+    }
+
     private var sourcesCard: some View {
         VStack(alignment: .leading, spacing: 12) {
             HStack {