feat: add thumbnail spec

src/lib/Image.svelte

@@ -2,8 +2,11 @@
 	type ImageStatus = 'loading' | 'loaded' | 'error';
 	import type { HTMLImgAttributes } from 'svelte/elements';
 
-	let { src, ...props }: HTMLImgAttributes = $props();
+	let { src, fallback, ...props }: HTMLImgAttributes & { fallback?: string } = $props();
 	let status: ImageStatus = $state('loading');
+	let useFallback = $state(false);
+
+	const currentSrc = $derived(useFallback && fallback ? fallback : src);
 </script>
 
 {#if status === 'loading'}
@@ -11,11 +14,18 @@
 {/if}
 
 <img
-	{src}
+	src={currentSrc}
 	hidden={status === 'loading'}
 	onload={() => {
 		status = 'loaded';
 	}}
-	onerror={() => (status = 'error')}
+	onerror={() => {
+		if (!useFallback && fallback) {
+			useFallback = true;
+			status = 'loading';
+		} else {
+			status = 'error';
+		}
+	}}
 	{...props}
 />