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/LockScreenView.swift

@@ -1616,10 +1616,12 @@ struct LockScreenView: View {
         ZStack {
             // Themed background
             backgroundView
+                .accessibilityHidden(true)
 
             // Floating particles (Aurora only)
             if lockScreenStyle == .aurora {
                 FloatingParticlesView()
+                    .accessibilityHidden(true)
             }
 
             // Main content
@@ -1630,6 +1632,7 @@ struct LockScreenView: View {
                 centralElement
                     .opacity(showContent ? 1 : 0)
                     .scaleEffect(showContent ? 1 : 0.8)
+                    .accessibilityHidden(true)
 
                 Spacer()
                     .frame(height: 50)
@@ -1649,6 +1652,7 @@ struct LockScreenView: View {
                 .multilineTextAlignment(.center)
                 .opacity(showContent ? 1 : 0)
                 .offset(y: showContent ? 0 : 20)
+                .accessibilityElement(children: .combine)
 
                 Spacer()
                     .frame(height: 16)
@@ -1666,6 +1670,8 @@ struct LockScreenView: View {
                     .opacity(showContent ? 1 : 0)
                     .offset(y: showContent ? 0 : 30)
                     .padding(.horizontal, 32)
+                    .accessibilityLabel("Unlock")
+                    .accessibilityHint("Double tap to authenticate with \(authManager.biometricName)")
 
                 // Passcode button
                 if authManager.canUseDevicePasscode {
@@ -1684,6 +1690,8 @@ struct LockScreenView: View {
                     .disabled(authManager.isAuthenticating)
                     .padding(.top, 16)
                     .opacity(showContent ? 1 : 0)
+                    .accessibilityLabel("Use device passcode")
+                    .accessibilityHint("Double tap to authenticate with your device passcode")
                 }
 
                 Spacer()