Fix text disappearing when unchecking curved text

- Initialize curvedPath: null when creating text elements
- Ensures all text elements have the curvedPath property from creation
- Prevents text loss when toggling curved text on/off
- Text content is now properly preserved when switching between curved and straight

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

src/components/DesignCanvas.tsx

@@ -630,6 +630,7 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
         fontFamily: 'Fraunces',
         fontSize: 28,
         fill: '#17181C',
+        curvedPath: null,
       },
     ]);
     setSelectedId(id);