Display submission success/error message after design submission

The submissionMessage state was being set but never displayed in the UI.
Added message display to show feedback when user submits for approval.

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

src/components/DesignCanvas.tsx

@@ -1542,6 +1542,7 @@ export default function DesignCanvas({ product, isAdminEditing = false }: { prod
           {elementsFront.length === 0 && elementsBack.length === 0 && !approvalStatus && (
             <p className="text-xs text-splash-pink">Add text or images to personalize your design before submitting</p>
           )}
+          {submissionMessage && <p className="text-sm font-medium">{submissionMessage}</p>}
           {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
         </div>
       </div>