Demo animation improvements: grid visibility, smooth scroll, fluid fill

- Fix grid visibility: show gray background cells always, animate mood
  colors on top with scale 2x->1x effect
- Add smooth auto-scroll for MonthView: starts after first month is 50%
  filled, scrolls at constant speed to match fill rate
- Remove pause between months for fluid continuous animation
- Add currentAnimatingMonthIndex tracking to DemoAnimationManager

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Shared/Views/YearView/YearView.swift

@@ -757,26 +757,40 @@ struct DemoYearHeatmapCell: View {
     @State private var randomMood: Mood = .great
 
     var body: some View {
-        RoundedRectangle(cornerRadius: 2)
-            .fill(cellColor)
-            .aspectRatio(1, contentMode: .fit)
-            .scaleEffect(cellScale)
-            .opacity(cellOpacity)
-            .animation(.spring(response: 0.35, dampingFraction: 0.5), value: isVisible)
-            .onAppear {
-                // Generate random mood once when cell appears
-                randomMood = DemoAnimationManager.randomPositiveMood()
+        ZStack {
+            // Background: Gray grid cell always visible at 1x scale
+            RoundedRectangle(cornerRadius: 2)
+                .fill(Color.gray.opacity(0.3))
+                .aspectRatio(1, contentMode: .fit)
+
+            // Foreground: Animated mood color that scales 2x -> 1x
+            if demoManager.isDemoMode {
+                RoundedRectangle(cornerRadius: 2)
+                    .fill(moodTint.color(forMood: randomMood))
+                    .aspectRatio(1, contentMode: .fit)
+                    .scaleEffect(isAnimated ? 1.0 : 2.0)
+                    .opacity(isAnimated ? 1.0 : 0.0)
+                    .animation(.spring(response: 0.3, dampingFraction: 0.7), value: isAnimated)
+            } else {
+                // Normal mode - just show the actual color
+                RoundedRectangle(cornerRadius: 2)
+                    .fill(normalCellColor)
+                    .aspectRatio(1, contentMode: .fit)
             }
+        }
+        .onAppear {
+            // Generate random mood once when cell appears
+            randomMood = DemoAnimationManager.randomPositiveMood()
+        }
     }
 
-    /// Whether this cell should be visible in demo mode
-    /// For year view, we animate column by column (left to right), then row by row within each column
-    private var isVisible: Bool {
+    /// Whether this cell has been animated (filled with color)
+    private var isAnimated: Bool {
         if !demoManager.isDemoMode {
-            return true // Normal mode - always visible
+            return true // Normal mode - always show
         }
         if !demoManager.animationStarted {
-            return false // Demo mode but animation hasn't started - hide all
+            return false // Demo mode but animation hasn't started
         }
 
         return demoManager.isCellVisibleForYear(
@@ -788,28 +802,8 @@ struct DemoYearHeatmapCell: View {
         )
     }
 
-    private var cellScale: CGFloat {
-        isVisible ? 1.0 : 0.0
-    }
-
-    private var cellOpacity: Double {
-        isVisible ? 1.0 : 0.0
-    }
-
-    private var cellColor: Color {
-        if demoManager.isDemoMode {
-            // In demo mode, show random positive mood colors
-            if !isVisible {
-                return Color.gray.opacity(0.1)
-            }
-            // Skip placeholder/empty cells
-            if color == Mood.placeholder.color || color == Mood.missing.color {
-                return Color.gray.opacity(0.1)
-            }
-            return moodTint.color(forMood: randomMood)
-        }
-
-        // Normal mode - use actual colors
+    /// Color for normal (non-demo) mode
+    private var normalCellColor: Color {
         if !isFiltered {
             return Color.gray.opacity(0.1)
         } else if color == Mood.placeholder.color || color == Mood.missing.color {