Fix 25 audit issues: memory leaks, concurrency, performance, accessibility

Address findings from comprehensive audit across 5 workstreams:

- Memory: Token-based DataController listeners (prevent closure leaks),
  static DateFormatters, ImageCache observer cleanup, MotionManager
  reference counting, FoundationModels dedup guard
- Concurrency: Replace Task.detached with Task in FeelsApp (preserve
  MainActor isolation), wrap WatchConnectivity handler in MainActor
- Performance: Cache sortedGroupedData in DayViewViewModel, cache demo
  data in MonthView/YearView, remove broken ReduceMotionModifier
- Accessibility: VoiceOver support for LockScreen, DemoHeatmapCell
  labels, MonthCard button labels, InsightsView header traits,
  Smart Invert protection on neon headers

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

Shared/Views/InsightsView/InsightsView.swift

@@ -237,6 +237,7 @@ struct InsightsSectionView: View {
                 .padding(.vertical, 14)
             }
             .buttonStyle(.plain)
+            .accessibilityAddTraits(.isHeader)
 
             // Insights List (collapsible)
             if isExpanded {
@@ -415,6 +416,7 @@ struct InsightCardView: View {
             RoundedRectangle(cornerRadius: 12)
                 .fill(colorScheme == .dark ? Color(.systemGray5) : Color(.systemGray6))
         )
+        .accessibilityElement(children: .combine)
     }
 }