fix: _roundRect arcTo missing parameter in ContentSharePoster

- Line 371 arcTo had only 4 args, needs 5 (x1,y1,x2,y2,radius)
- Caused bottom-left rounded rect rendering failure on all canvas shapes

cfc-frontend/components/ContentSharePoster.vue

@@ -368,7 +368,7 @@ export default {
       ctx.moveTo(x + r, y)
       ctx.arcTo(x + w, y, x + w, y + r, r)
       ctx.arcTo(x + w, y + h, x + w - r, y + h, r)
-      ctx.arcTo(x, y + h, y + h - r, r)
+      ctx.arcTo(x, y + h, x, y + h - r, r)
       ctx.arcTo(x, y, x + r, y, r)
       ctx.closePath()
     },