M3.1 follow-up: ShareCard grade-B billboard → Design white; drop stale comment hexes

The share/OG card (a billboard) still painted grade B blue (#4A9EFF) on canvas;
Design's B is neutral-bright white. Canvas needs a literal, so #F0F0F0 direct.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/components/ShareCard.tsx

@@ -23,7 +23,7 @@ const SPORT_COLOR: Record<ShareCardProps['sport'], string> = {
 function gradeColor(grade: string): string {
   const g = (grade || '').trim().toUpperCase().charAt(0);
   if (g === 'A') return '#00C896';
-  if (g === 'B') return '#4A9EFF';
+  if (g === 'B') return '#F0F0F0';
   if (g === 'C') return '#FFB347';
   return '#FF6B6B';
 }