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/DemoAnimationManager.swift

@@ -29,8 +29,8 @@ final class DemoAnimationManager: ObservableObject {
     /// Total animation duration for filling all cells in one month
     let monthAnimationDuration: TimeInterval = 1.5
 
-    /// Delay between months
-    let monthDelay: TimeInterval = 0.3
+    /// Delay between months (0 for fluid continuous animation)
+    let monthDelay: TimeInterval = 0.0
 
     /// Delay between each cell in year view
     let yearCellDelay: TimeInterval = 0.05
@@ -116,6 +116,23 @@ final class DemoAnimationManager: ObservableObject {
         return Double(monthIndex) * (monthAnimationDuration + monthDelay)
     }
 
+    /// Returns the current month index being animated (for auto-scrolling)
+    var currentAnimatingMonthIndex: Int {
+        guard animationStarted else { return 0 }
+        let monthDuration = monthAnimationDuration + monthDelay
+        return Int(animationProgress / monthDuration)
+    }
+
+    /// Returns the approximate row being animated within current month (0-5 typically)
+    func currentAnimatingRow(totalRows: Int, totalColumns: Int) -> Int {
+        guard animationStarted else { return 0 }
+        let monthDuration = monthAnimationDuration + monthDelay
+        let progressInMonth = animationProgress.truncatingRemainder(dividingBy: monthDuration)
+        let normalizedProgress = min(1.0, progressInMonth / monthAnimationDuration)
+        let cellIndex = Int(normalizedProgress * Double(totalRows * totalColumns))
+        return cellIndex / totalColumns
+    }
+
     /// Check if a cell should be visible based on current animation progress
     /// For MonthView: animates cells within each month from top-left to bottom-right
     /// - Parameters: