Fix dashboard image disk usage calculation (#1461)

* fix dashboard image disk usage calculation

* preserve image size fallback behavior

tests/docker-disk-usage-core.test.ts

@@ -0,0 +1,53 @@
+// @ts-expect-error -- bun:test is a runtime built-in with no types installed
+import { describe, expect, test } from 'bun:test';
+import { getImageDiskUsageTotalSize } from '../src/lib/server/docker-disk-usage-core';
+
+describe('getImageDiskUsageTotalSize', () => {
+	test('uses Docker aggregate size instead of summing virtual image sizes', () => {
+		const diskUsage = {
+			LayersSize: 70_326_328_267,
+			Images: [
+				{ Size: 189_295_958_800 },
+				{ Size: 82_498_083_216 }
+			]
+		};
+
+		expect(getImageDiskUsageTotalSize(diskUsage)).toBe(70_326_328_267);
+	});
+
+	test('prefers the current ImageUsage.TotalSize response field', () => {
+		expect(getImageDiskUsageTotalSize({
+			ImageUsage: { TotalSize: 42 },
+			LayersSize: 99
+		})).toBe(42);
+	});
+
+	test('supports the legacy LayersSize response field', () => {
+		expect(getImageDiskUsageTotalSize({ LayersSize: 42 })).toBe(42);
+	});
+
+	test('preserves zero as a valid aggregate size', () => {
+		expect(getImageDiskUsageTotalSize({
+			ImageUsage: { TotalSize: 0 },
+			LayersSize: 99
+		})).toBe(0);
+		expect(getImageDiskUsageTotalSize({ LayersSize: 0 })).toBe(0);
+	});
+
+	test('falls back from an invalid current value to a valid legacy value', () => {
+		expect(getImageDiskUsageTotalSize({
+			ImageUsage: { TotalSize: Number.NaN },
+			LayersSize: 42
+		})).toBe(42);
+	});
+
+	test('returns null when no trustworthy aggregate is available', () => {
+		expect(getImageDiskUsageTotalSize(undefined)).toBeNull();
+		expect(getImageDiskUsageTotalSize({})).toBeNull();
+		expect(getImageDiskUsageTotalSize({ LayersSize: -1 })).toBeNull();
+		expect(getImageDiskUsageTotalSize({
+			ImageUsage: { TotalSize: '42' },
+			LayersSize: null
+		})).toBeNull();
+	});
+});