Fix: Ensure cart is completely cleared after payment success

ClearCartOnMount now explicitly deletes the IndexedDB storage in addition
to clearing the in-memory state. This prevents the cart from being
restored from IndexedDB on subsequent page loads after payment completes.

Fixes issue where cart items remained in bag after successful checkout.

.claude/settings.local.json

@@ -72,7 +72,8 @@
       "Bash(git commit -m 'Fix: Safely parse designJson in admin order detail page *)",
       "Bash(xargs grep -l \"design-spec\")",
       "Bash(git commit -m 'Improve: Design specification sheet now shows previews and info *)",
-      "Bash(git commit -m 'Fix: Handle undefined designElements in design spec SVG rendering *)"
+      "Bash(git commit -m 'Fix: Handle undefined designElements in design spec SVG rendering *)",
+      "Bash(git commit -m 'Fix: Ensure cart is completely cleared after payment success *)"
     ]
   }
 }