fix: reduce memory footprint — Canvas for dot grid, scope PreferenceKey

DotGridProgress replaced GeometryReader + 200+ Circle view nodes with a
single Canvas draw call. Canvas receives its own size parameter so no
GeometryReader is needed; SwiftUI interpolates the progress value through
the .animation modifier for smooth fill transitions.

FloatingTabState ScrollYKey made private to TabBarScrollModifier to limit
its PreferenceKey scope and prevent unintended propagation across unrelated
subtrees.

KisaniCal/Views/TodayView.swift

@@ -1107,39 +1107,29 @@ struct DotGridProgress: View {
     private let rows             = 7
 
     var body: some View {
-        GeometryReader { geo in
-            let cols   = max(1, Int((geo.size.width + gap) / (dotSize + gap)))
+        Canvas { ctx, size in
+            let step = dotSize + gap
+            let cols = max(1, Int((size.width + gap) / step))
             let total  = cols * rows
             let filled = Int(Double(total) * min(max(progress, 0), 1))
 
-            VStack(alignment: .leading, spacing: gap) {
-                ForEach(0..<rows, id: \.self) { row in
-                    DotRow(cols: cols, rowStart: row * cols, filled: filled,
-                           dotSize: dotSize, gap: gap, cs: cs)
+            for row in 0..<rows {
+                for col in 0..<cols {
+                    let idx = row * cols + col
+                    let rect = CGRect(
+                        x: CGFloat(col) * step,
+                        y: CGFloat(row) * step,
+                        width: dotSize, height: dotSize
+                    )
+                    ctx.fill(
+                        Path(ellipseIn: rect),
+                        with: .color(idx < filled ? AppColors.green : AppColors.green.opacity(0.12))
+                    )
                 }
             }
-            .animation(KisaniSpring.entrance, value: filled)
-        }
-        .frame(height: CGFloat(rows) * dotSize + CGFloat(rows - 1) * gap)
-    }
-}
-
-struct DotRow: View {
-    let cols: Int
-    let rowStart: Int
-    let filled: Int
-    let dotSize: CGFloat
-    let gap: CGFloat
-    let cs: ColorScheme
-
-    var body: some View {
-        HStack(spacing: gap) {
-            ForEach(0..<cols, id: \.self) { col in
-                let isFilled = (rowStart + col) < filled
-                Circle()
-                    .fill(isFilled ? AppColors.green : AppColors.green.opacity(0.12))
-                    .frame(width: dotSize, height: dotSize)
-            }
         }
+        .frame(maxWidth: .infinity)
+        .frame(height: CGFloat(rows) * (dotSize + gap) - gap)
+        .animation(KisaniSpring.entrance, value: progress)
     }
 }