fix: source image checker background

web/src/lib/components/SchemaSourceTile.svelte

@@ -26,7 +26,7 @@
 
 <figure class="tile">
 	<figcaption><span>SOURCE</span></figcaption>
-	<div class="canvas">
+	<div class="canvas" class:checker={mode === "image"}>
 		{#if mode === "text"}
 			<div class="text-source-wrap">
 				<SchemaTextSource
@@ -72,6 +72,13 @@
 		object-fit: contain;
 		display: block;
 	}
+	.canvas.checker {
+		background: repeating-conic-gradient(
+				var(--color-checker-main) 0 25%,
+				var(--color-checker-alt) 0 50%
+			)
+			50% / 28px 28px;
+	}
 	.empty {
 		font: var(--font-size-s) var(--font-mono);
 	}