feat(frontend): ImageVariant component and WASM encoder workers

Add client-side multi-format image encoding via Web Workers using @jsquash and @discourse/jxl libraries. ImageVariant Svelte component renders <picture> elements with format-aware fallback. Vite configured for WASM asset inclusion and encoder dependency exclusion. Portfolio page updated to use multi-format URLs.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/lib/components/ui/ImageVariant.svelte

@@ -0,0 +1,41 @@
+<script lang="ts">
+	interface ThumbURLs {
+		avif: string;
+		webp: string;
+		jpg: string;
+	}
+
+	interface FullURLs {
+		avif: string;
+		webp: string;
+		jpg: string;
+		jxl?: string;
+	}
+
+	let { urls, type = "thumb", alt = "", class: className = "", ...imgProps }: {
+		urls: ThumbURLs | FullURLs;
+		type?: "thumb" | "full";
+		alt?: string;
+		class?: string;
+		[key: string]: unknown;
+	} = $props();
+
+	const fallbackSrc = $derived(urls.jpg || urls.webp || urls.avif || "");
+	const hasJxl = $derived(type === "full" && "jxl" in urls && (urls as FullURLs).jxl);
+</script>
+
+<picture>
+	{#if urls.avif}
+		<source srcset={urls.avif} type="image/avif" />
+	{/if}
+	{#if urls.webp}
+		<source srcset={urls.webp} type="image/webp" />
+	{/if}
+	{#if urls.jpg}
+		<source srcset={urls.jpg} type="image/jpeg" />
+	{/if}
+	{#if hasJxl}
+		<source srcset={(urls as FullURLs).jxl} type="image/jxl" />
+	{/if}
+	<img src={fallbackSrc} {alt} class={className} {...imgProps} />
+</picture>