fix: crossfade simplifié — canvas toujours visible, seul l'overlay se dissout

Le canvas reste toujours à opacity 1. Quand les transactions changent :
1. Capture le canvas dans l'overlay img (snap à opacity 1 sans transition)
2. Met à jour le canvas en dessous
3. Double rAF pour laisser Leaflet.heat redessiner
4. Dissout l'overlay de 1→0 en 500ms via CSS transition

Élimine le double-affichage et les conflits de transition canvas/overlay.

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

src/components/HeatMap.tsx

@@ -91,12 +91,7 @@ export function HeatMap({ transactions }: HeatMapProps) {
       return;
     }
 
-    // 1. Cancel any running overlay transition instantly (flush only the overlay)
+    // 1. Freeze previous frame in overlay (canvas still at opacity 1)
-    overlay.style.transition = 'none';
-    overlay.style.opacity = '0';
-    void overlay.offsetWidth; // flush overlay only — avoids double-paint with canvas
-
-    // 2. Capture current frame (canvas still visible at opacity 1)
     try {
       overlay.src = canvas.toDataURL();
     } catch {
@@ -104,22 +99,19 @@ export function HeatMap({ transactions }: HeatMapProps) {
       return;
     }
 
-    // 3. In one paint batch: hide canvas + show overlay (Frame A moves from canvas to overlay)
+    // 2. Snap overlay visible — cancel any running transition first
-    canvas.style.transition = 'none';
+    overlay.style.transition = 'none';
-    canvas.style.opacity = '0';
+    void overlay.offsetWidth; // flush
     overlay.style.opacity = '1';
 
-    // 4. Update heatmap (invisible: canvas at opacity 0)
+    // 3. Update canvas underneath (canvas always at opacity 1, covered by overlay)
     update();
 
-    // 5. Simultaneous crossfade: overlay fades out, canvas fades in
+    // 4. After Leaflet redraws: dissolve overlay away, revealing new frame
     const raf = requestAnimationFrame(() => {
       requestAnimationFrame(() => {
-        const DURATION = '0.5s ease-in-out';
+        overlay.style.transition = 'opacity 0.5s ease-in-out';
-        overlay.style.transition = `opacity ${DURATION}`;
         overlay.style.opacity = '0';
-        canvas.style.transition = `opacity ${DURATION}`;
-        canvas.style.opacity = '1';
       });
     });