fix: Create a distinct containing block for each breakpoint canvas

To avoid positioned elements leaking out of the container when all breakpoints are visible.

Previously, whole canvas container was getting considered as the containing block for all elements which is why any fixed element with relative widths were off when all the breakpoints were switched on.

closes: #287

frontend/src/components/BuilderCanvas.vue

@@ -37,7 +37,7 @@
 				</div>
 			</div>
 			<div
-				class="canvas relative flex h-full rounded-md bg-surface-white shadow-2xl"
+				class="canvas relative flex h-full rounded-md bg-surface-white shadow-2xl contain-layout"
 				:style="{
 					...canvasStyles,
 					background: canvasProps.background,