fix(downloads): harden enqueue lifecycle races

Reject superseded enqueue generations in the queue manager and coordinate frontend dispatch, pause, removal, and property mutations.

src/store/downloadStore.test.ts

@@ -1,8 +1,15 @@
-import { beforeEach, describe, expect, it } from 'vitest';
-import { useDownloadProgressStore } from './downloadStore';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { initDownloadListener, useDownloadProgressStore } from './downloadStore';
+import * as ipc from '../ipc';
+
+vi.mock('../ipc', () => ({
+  invokeCommand: vi.fn(),
+  listenEvent: vi.fn(),
+}));
 
 describe('useDownloadProgressStore', () => {
   beforeEach(() => {
+    vi.clearAllMocks();
     useDownloadProgressStore.setState({ progressMap: {} });
   });
 
@@ -20,4 +27,22 @@ describe('useDownloadProgressStore', () => {
 
     expect(useDownloadProgressStore.getState().progressMap).toEqual({});
   });
+
+  it('shares listener setup across overlapping consumers and tears down after the last release', async () => {
+    const unlisten = vi.fn();
+    vi.mocked(ipc.listenEvent).mockResolvedValue(unlisten);
+
+    const first = initDownloadListener();
+    const second = initDownloadListener();
+
+    expect(ipc.listenEvent).toHaveBeenCalledTimes(3);
+
+    const releaseFirst = await first;
+    const releaseSecond = await second;
+    releaseFirst();
+    expect(unlisten).not.toHaveBeenCalled();
+
+    releaseSecond();
+    expect(unlisten).toHaveBeenCalledTimes(3);
+  });
 });