Add allocation evolution chart showing allocation changes over time

Adds a stacked area chart under the Allocation tab that displays how the
portfolio allocation percentages have changed across months.

Fixes #25

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

PortfolioJournal/ViewModels/ChartsViewModel.swift

@@ -103,6 +103,7 @@ class ChartsViewModel: ObservableObject {
     @Published var drawdownData: [(date: Date, drawdown: Double)] = []
     @Published var volatilityData: [(date: Date, volatility: Double)] = []
     @Published var predictionData: [Prediction] = []
+    @Published var allocationEvolutionData: [(date: Date, category: String, percentage: Double, color: String)] = []
 
     @Published var isLoading = false
     @Published var showingPaywall = false
@@ -340,6 +341,7 @@ class ChartsViewModel: ObservableObject {
             calculateCategoryEvolutionData(from: completedSnapshots, categories: categoriesForChart)
         case .allocation:
             calculateAllocationData(for: sources, breakdown: selectedBreakdown)
+            calculateAllocationEvolutionData(from: completedSnapshots)
         case .performance:
             let completedSnapshotsBySource = groupSnapshotsBySource(completedSnapshots)
             calculatePerformanceData(
@@ -587,6 +589,57 @@ class ChartsViewModel: ObservableObject {
         }
     }
 
+    private func calculateAllocationEvolutionData(from snapshots: [Snapshot]) {
+        let sortedSnapshots = snapshots.sorted { $0.date < $1.date }
+        let groupedByMonth = Dictionary(grouping: sortedSnapshots) { snapshot -> DateComponents in
+            Calendar.current.dateComponents([.year, .month], from: snapshot.date)
+        }
+
+        let sortedMonths = groupedByMonth.keys.sorted {
+            let d1 = ($0.year ?? 0) * 100 + ($0.month ?? 0)
+            let d2 = ($1.year ?? 0) * 100 + ($1.month ?? 0)
+            return d1 < d2
+        }
+
+        var result: [(date: Date, category: String, percentage: Double, color: String)] = []
+
+        for monthKey in sortedMonths {
+            guard let monthSnapshots = groupedByMonth[monthKey],
+                  let monthDate = Calendar.current.date(from: monthKey) else { continue }
+
+            var categoryTotals: [String: (value: Decimal, color: String)] = [:]
+            var sourceLatest: [UUID: Snapshot] = [:]
+
+            for snapshot in monthSnapshots {
+                guard let sourceId = snapshot.source?.id else { continue }
+                if let existing = sourceLatest[sourceId] {
+                    if snapshot.date > existing.date {
+                        sourceLatest[sourceId] = snapshot
+                    }
+                } else {
+                    sourceLatest[sourceId] = snapshot
+                }
+            }
+
+            for (_, snapshot) in sourceLatest {
+                let categoryName = snapshot.source?.category?.name ?? "Other"
+                let colorHex = snapshot.source?.category?.colorHex ?? "#6B7280"
+                let existing = categoryTotals[categoryName] ?? (value: 0, color: colorHex)
+                categoryTotals[categoryName] = (value: existing.value + snapshot.decimalValue, color: colorHex)
+            }
+
+            let total = categoryTotals.values.reduce(Decimal.zero) { $0 + $1.value }
+            guard total > 0 else { continue }
+
+            for (category, info) in categoryTotals.sorted(by: { $0.value.value > $1.value.value }) {
+                let percentage = NSDecimalNumber(decimal: info.value / total * 100).doubleValue
+                result.append((date: monthDate, category: category, percentage: percentage, color: info.color))
+            }
+        }
+
+        allocationEvolutionData = result
+    }
+
     private func completedMonthKeys(
         sources: [InvestmentSource],
         snapshots: [Snapshot],