Allow viewing evolution chart for individual or multiple sources across categories

Adds multi-source selection to the evolution chart source filter.
Users can now tap multiple sources to compare their evolution side by side,
regardless of which category they belong to.

Fixes #24

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

PortfolioJournal/ViewModels/ChartsViewModel.swift

@@ -86,6 +86,7 @@ class ChartsViewModel: ObservableObject {
     @Published var selectedChartType: ChartType = .evolution
     @Published var selectedCategory: Category?
     @Published var selectedSource: InvestmentSource?
+    @Published var selectedSourceIds: Set<UUID> = []
     @Published var selectedTimeRange: TimeRange = .year
     @Published var selectedAccount: Account?
     @Published var showAllAccounts = true
@@ -290,7 +291,12 @@ class ChartsViewModel: ObservableObject {
         }
 
         let sources: [InvestmentSource]
-        if let selectedSource {
+        if !selectedSourceIds.isEmpty {
+            sources = sourceRepository.sources.filter { source in
+                guard let id = source.id else { return false }
+                return selectedSourceIds.contains(id) && shouldIncludeSource(source)
+            }
+        } else if let selectedSource {
             sources = sourceRepository.sources.filter { $0.id == selectedSource.id && shouldIncludeSource($0) }
         } else if let category = category {
             sources = sourceRepository.fetchSources(for: category).filter { shouldIncludeSource($0) }