Show source value difference vs previous check-in (green/red coloring)

When a source has been updated this cycle, shows the value change from the
previous snapshot in green (increase) or red (decrease).

Fixes #21

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

PortfolioJournal/Views/Dashboard/MonthlyCheckInView.swift

@@ -437,6 +437,15 @@ struct MonthlyCheckInView: View {
                 ForEach(viewModel.sources, id: \.objectID) { source in
                     let latestSnapshot = source.latestSnapshot
                     let updatedThisCycle = isSnapshotInCurrentCycle(latestSnapshot)
+                    let snapshots = source.sortedSnapshotsByDateAscending
+                    let previousSnapshot: Snapshot? = {
+                        guard snapshots.count >= 2 else { return nil }
+                        return snapshots[snapshots.count - 2]
+                    }()
+                    let valueDiff: Decimal? = {
+                        guard let latest = latestSnapshot, let previous = previousSnapshot else { return nil }
+                        return latest.decimalValue - previous.decimalValue
+                    }()
                     Button {
                         if updatedThisCycle, let snapshot = latestSnapshot {
                             editingSnapshot = snapshot
@@ -459,6 +468,12 @@ struct MonthlyCheckInView: View {
 
                             Spacer()
 
+                            if let diff = valueDiff, updatedThisCycle {
+                                Text(diff >= 0 ? "+\(diff.compactCurrencyString)" : diff.compactCurrencyString)
+                                    .font(.caption.weight(.semibold))
+                                    .foregroundColor(diff >= 0 ? .positiveGreen : .negativeRed)
+                            }
+
                             Text(latestSnapshot?.date.relativeDescription ?? String(localized: "date_never"))
                                 .font(.caption)
                                 .foregroundColor(.secondary)