feat(publish): add shared media-image asset normalizer

Introduce a reusable helper that produces upload-ready blobs for
adapters that share the same image-asset semantics (dongchedi,
sohuhao, wangyihao, weixin-gzh, zol). Centralizes URL candidate
expansion and passthrough mime detection so each adapter focuses on
its own publish protocol.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop-client/src/main/adapters/media-image.test.ts

@@ -0,0 +1,44 @@
+import { Buffer } from "node:buffer";
+
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("electron", () => ({
+  nativeImage: {
+    createFromBuffer: () => ({
+      isEmpty: () => false,
+      getSize: () => ({ width: 1, height: 1 }),
+      crop: () => ({
+        isEmpty: () => false,
+        getSize: () => ({ width: 1, height: 1 }),
+        toPNG: () => Buffer.from([]),
+      }),
+      toPNG: () => Buffer.from([]),
+    }),
+  },
+}));
+
+vi.mock("../transport/api-client", () => ({
+  resolveDesktopApiURL: (path: string) => `http://127.0.0.1:8080${path}`,
+}));
+
+import {
+  buildAssetURLCandidates,
+  getCenteredCropRect,
+} from "./media-image";
+
+describe("media image helpers", () => {
+  it("resolves API asset URLs through the desktop API base and requests png output", () => {
+    expect(buildAssetURLCandidates("/api/public/assets/12").at(0)).toBe(
+      "http://127.0.0.1:8080/api/public/assets/12?format=png",
+    );
+  });
+
+  it("centers crop rectangles at the requested ratio", () => {
+    const rect = getCenteredCropRect(1600, 1000, 4 / 3);
+
+    expect(rect.x).toBeCloseTo(133.333, 3);
+    expect(rect.y).toBeCloseTo(0);
+    expect(rect.width).toBeCloseTo(1333.333, 3);
+    expect(rect.height).toBeCloseTo(1000);
+  });
+});