Fix dashboard image disk usage calculation (#1461)

* fix dashboard image disk usage calculation

* preserve image size fallback behavior

src/lib/server/docker-disk-usage-core.ts

@@ -0,0 +1,32 @@
+export interface DockerDiskUsageLike {
+	LayersSize?: unknown;
+	ImageUsage?: {
+		TotalSize?: unknown;
+	} | null;
+	Images?: Array<{
+		Size?: unknown;
+	}> | null;
+}
+
+function asNonNegativeFiniteNumber(value: unknown): number | null {
+	return typeof value === 'number' && Number.isFinite(value) && value >= 0
+		? value
+		: null;
+}
+
+/**
+ * Return Docker's aggregate image disk usage without double-counting shared layers.
+ *
+ * `Images[].Size` is each image's virtual size (shared + unique), so adding those
+ * values overstates disk usage whenever images share layers. Docker exposes the
+ * deduplicated total as `ImageUsage.TotalSize` in the current API and as
+ * `LayersSize` in the legacy response shape.
+ */
+export function getImageDiskUsageTotalSize(
+	diskUsage: DockerDiskUsageLike | null | undefined
+): number | null {
+	const currentTotal = asNonNegativeFiniteNumber(diskUsage?.ImageUsage?.TotalSize);
+	if (currentTotal !== null) return currentTotal;
+
+	return asNonNegativeFiniteNumber(diskUsage?.LayersSize);
+}