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/MoodLogger.swift

@@ -19,6 +19,7 @@ final class MoodLogger {
 
     /// Key for tracking the last date side effects were applied
     private static let lastSideEffectsDateKey = "lastSideEffectsAppliedDate"
+    private static let sideEffectsDateFormatter = ISO8601DateFormatter()
 
     private init() {}
 
@@ -248,14 +249,14 @@ final class MoodLogger {
 
     /// Mark that side effects have been applied for a given date
     private func markSideEffectsApplied(for date: Date) {
-        let dateString = ISO8601DateFormatter().string(from: Calendar.current.startOfDay(for: date))
+        let dateString = Self.sideEffectsDateFormatter.string(from: Calendar.current.startOfDay(for: date))
         GroupUserDefaults.groupDefaults.set(dateString, forKey: Self.lastSideEffectsDateKey)
     }
 
     /// Check if side effects have been applied for a given date
     private func sideEffectsApplied(for date: Date) -> Bool {
         guard let lastDateString = GroupUserDefaults.groupDefaults.string(forKey: Self.lastSideEffectsDateKey),
-              let lastDate = ISO8601DateFormatter().date(from: lastDateString) else {
+              let lastDate = Self.sideEffectsDateFormatter.date(from: lastDateString) else {
             return false
         }