Feat: Add checkout options modal after adding design to bag

- Show modal after design is approved and added to bag
- Offer two options: 'Go to Checkout' and 'Keep Shopping'
- 'Go to Checkout' navigates to /checkout
- 'Keep Shopping' closes the modal and lets customer continue

Improved UX - no more confusing confirmation dialogs, just
a clean modal with clear next steps.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.claude/settings.local.json

@@ -166,7 +166,8 @@
       "Bash(git commit -m 'Fix: Show approved designs in customer account so they can order *)",
       "Bash(git commit -m 'Fix: Link approved designs to review page, not personalization *)",
       "Bash(git commit -m 'Feat: Show estimated delivery date on design review page *)",
-      "Bash(git commit -m 'Fix: Approve & Add to Bag button not working *)"
+      "Bash(git commit -m 'Fix: Approve & Add to Bag button not working *)",
+      "Bash(git commit -m 'Feat: Add checkout options modal after adding design to bag *)"
     ]
   }
 }