Fix: Handle undefined designElements in design spec SVG rendering

The SVG element markers were trying to map over designElements without
checking if it was defined first. Now safely checks for array existence
before calling .map() to prevent errors when generating spec sheets.

src/app/api/orders/[id]/design-spec/route.ts

@@ -126,10 +126,10 @@ export async function GET(req: Request, { params }: { params: { id: string } })
                 <text x="${printAreaStartX - 25}" y="${printAreaStartY + 15}" font-size="12" fill="green" font-weight="bold">Y (0cm)</text>
 
                 <!-- Design elements with markers -->
-                ${designElements.map((el, idx) => `
+                ${designElements && designElements.length > 0 ? designElements.map((el, idx) => `
                   <circle cx="${el.x}" cy="${el.y}" r="5" fill="orange" opacity="0.9" stroke="white" stroke-width="1"/>
                   <text x="${el.x + 10}" y="${el.y - 10}" font-size="12" fill="orange" font-weight="bold" font-family="monospace">E${idx + 1}</text>
-                `).join('')}
+                `).join('') : ''}
               </svg>
             </div>
             <div style="margin-top: 15px; font-size: 12px; color: #666; text-align: left; max-width: 400px;">