feat(viewer): UI-02 heart particles, interaction notices, share and download

Task 5 (V05 §5.4/§6): decorative heart particles on the lightbox stage
(local like clicks + others' WS bursts for the current photo), compact
cross-photo interaction notices above the action bar (click to locate),
navigator.share-first share orchestration with clipboard fallback for
album and photo deep links (lang param), and download failures attributed
to the photo that triggered them. Self-echo dedup stays in the realtime
router (own batches never produce particles/notices); particles cap at
12 with visibility/unmount cleanup and are suppressed under reduced
motion; notices cap at 3 with ~4s auto-dismiss and throttled SR summary.

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/viewer/src/__tests__/download.spec.ts

@@ -1,5 +1,5 @@
 import { flushPromises, mount, type VueWrapper } from '@vue/test-utils';
-import { defineComponent, ref } from 'vue';
+import { defineComponent, nextTick, ref } from 'vue';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import {
   ApiError,
@@ -89,9 +89,10 @@ function spyWindowOpen() {
 }
 let openSpy: ReturnType<typeof spyWindowOpen>;
 
-/** 宿主实例形态：setup 暴露 albumRef（ref 经代理解包）+ download 控制器。 */
+/** 宿主实例形态：setup 暴露 albumRef/currentRef（ref 经代理解包）+ download 控制器。 */
 interface HostVm {
   albumRef: PublicAlbum | null;
+  currentRef: string;
   download: DownloadController;
 }
 
@@ -112,7 +113,7 @@ function mountLightbox(album: PublicAlbum): VueWrapper & { vm: HostVm } {
         album: () => albumRef.value,
         photoId: () => current.value,
       });
-      return { sequence, photoId: current, download, albumRef };
+      return { sequence, photoId: current, currentRef: current, download, albumRef };
     },
     template: `<PhotoLightbox v-model:photo-id="photoId" :sequence="sequence" :download="download" />`,
   });
@@ -251,6 +252,31 @@ describe('photo download (P4a Task 4)', () => {
     expect(openSpy).not.toHaveBeenCalled();
   });
 
+  it('attributes an async failure to the photo that triggered the download, not the current one', async () => {
+    // 在途请求挂起期间切图：错误归属触发时的 photoId（UI-02 §5.4）。
+    let rejectDownload!: (error: Error) => void;
+    mockedGetPhotoDownload.mockReset().mockImplementation(
+      () =>
+        new Promise((_resolve, reject) => {
+          rejectDownload = reject;
+        }),
+    );
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.download-main').trigger('click'); // p1 触发
+    wrapper.vm.currentRef = 'p2'; // 请求在途时切到 p2
+    rejectDownload(new Error('boom'));
+    await flushPromises();
+
+    expect(wrapper.vm.download.failedFor).toBe('p1');
+    expect(wrapper.find('.download-failed').exists()).toBe(false); // p2 不背锅
+
+    wrapper.vm.currentRef = 'p1'; // 切回 p1：错误提示可见
+    await nextTick();
+    expect(wrapper.find('.download-failed[role="alert"]').text()).toContain('下载失败');
+  });
+
   it('DOWNLOAD_DISABLED surfaces the dedicated localized notice (T15 漏译修复)', async () => {
     // 相册中途关闭下载开关（载荷仍开但端点 403）→ 专属文案而非通用失败。
     mockedGetPhotoDownload.mockReset().mockRejectedValue(