feat(builder): shared asset upload/list util

Extract uploadAsset/listAssets into utils/assets.ts, lifting the exact
uploadToWhp body and list_assets fetch pattern already duplicated across
shared.tsx/ImageBlock/Logo/Navbar/etc. shared.tsx's uploadToWhp is now a
thin re-export (`export const uploadToWhp = uploadAsset`) so its ~6
existing callers are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/right/styles/shared.uploadToWhp.test.ts

@@ -0,0 +1,21 @@
+import { describe, test, expect, vi, afterEach } from 'vitest';
+import { uploadToWhp } from './shared';
+import { uploadAsset } from '../../../utils/assets';
+
+describe('shared.uploadToWhp', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    delete (window as any).WHP_CONFIG;
+  });
+
+  test('is a thin re-export of uploadAsset (same function reference)', () => {
+    expect(uploadToWhp).toBe(uploadAsset);
+  });
+
+  test('still works standalone (no WHP_CONFIG -> blob: URL) for existing callers', async () => {
+    (URL as any).createObjectURL = vi.fn(() => 'blob:mock-url');
+    const file = new File(['x'], 'photo.png', { type: 'image/png' });
+    const url = await uploadToWhp(file);
+    expect(url).toBe('blob:mock-url');
+  });
+});