fix: vrai crossfade simultané — canvas masqué puis fade in+out en parallèle

Canvas caché (opacity 0) avant update → overlay (frame A) fade out
et canvas (frame B) fade in simultanément sur 500ms.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/HeatMap.tsx

@@ -91,25 +91,33 @@ export function HeatMap({ transactions }: HeatMapProps) {
       return;
     }
 
-    // Freeze current frame in the overlay
+    // 1. Hide canvas instantly (no transition)
+    canvas.style.transition = 'none';
+    canvas.style.opacity = '0';
+
+    // 2. Freeze current frame in the overlay
     try {
       overlay.src = canvas.toDataURL();
     } catch {
       // canvas tainted (shouldn't happen with heatmap-only canvas)
+      canvas.style.opacity = '1';
       update();
       return;
     }
     overlay.style.transition = 'none';
     overlay.style.opacity = '1';
 
-    // Update heatmap underneath immediately
+    // 3. Update heatmap (invisible: canvas still at opacity 0)
     update();
 
-    // Then dissolve the overlay away
+    // 4. Simultaneous crossfade: overlay fades out, canvas fades in
     const raf = requestAnimationFrame(() => {
       requestAnimationFrame(() => {
-        overlay.style.transition = 'opacity 0.5s ease-in-out';
+        const DURATION = '0.5s ease-in-out';
+        overlay.style.transition = `opacity ${DURATION}`;
         overlay.style.opacity = '0';
+        canvas.style.transition = `opacity ${DURATION}`;
+        canvas.style.opacity = '1';
       });
     });