fix: add missing fmtBytes helper in image-gen page

Fixes CI type-check failure: fmtBytes was used on line 592 to format
the reference image file size but was never defined.

ui/src/routes/admin/image-gen/+page.svelte

@@ -329,6 +329,12 @@
 		if (input) input.value = '';
 	}
 
+	function fmtBytes(bytes: number): string {
+		if (bytes < 1024) return `${bytes} B`;
+		if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+		return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+	}
+
 	// ── Selected model info ──────────────────────────────────────────────────────
 	let selectedModelInfo = $derived(models.find((m) => m.id === selectedModel) ?? null);
 	let refWarning = $derived(