fix: force reflow avant reset des transitions CSS du crossfade

Sans forcer un reflow, le browser ignore transition:none et applique
encore l'ancienne transition — causant un bug visuel sur la 1ère frame.
void canvas.offsetWidth flush les styles en attente.

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

src/components/HeatMap.tsx

@@ -91,20 +91,20 @@ export function HeatMap({ transactions }: HeatMapProps) {
       return;
     }
 
-    // 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;
     }
+
+    // 1. Reset transitions instantly — force reflow to flush any running transition
+    canvas.style.transition = 'none';
     overlay.style.transition = 'none';
+    void canvas.offsetWidth; // force reflow
+    canvas.style.opacity = '0';
     overlay.style.opacity = '1';
 
     // 3. Update heatmap (invisible: canvas still at opacity 0)