feat(queue): implement concurrent deduplication and safe backend detachment

- Add backendRegisteredIds and backendDispatchPromises for single dispatch enforcement

- Add detach_download_for_reconfigure to safely modify properties of active downloads

- Add applyProperties logic handling completed, failed, paused, and active queues

- Add extractValidDownloadUrls and fix multi-url paste handling

- Setup vitest and add useDownloadStore unit tests for backend registration lifecycle

src/store/useDownloadStore.test.ts

@@ -0,0 +1,94 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { useDownloadStore } from './useDownloadStore';
+import * as ipc from '../ipc';
+
+vi.mock('../ipc', () => ({
+  invokeCommand: vi.fn(),
+}));
+
+// Mock window.__TAURI_INTERNALS__ and log to prevent errors
+vi.mock('@tauri-apps/plugin-log', () => ({
+  info: vi.fn(),
+  error: vi.fn(),
+}));
+
+vi.mock('@tauri-apps/plugin-store', () => {
+  return {
+    LazyStore: class {
+      get = vi.fn().mockResolvedValue([]);
+      set = vi.fn();
+      save = vi.fn();
+    }
+  };
+});
+
+vi.mock('./useSettingsStore', () => ({
+  useSettingsStore: {
+    getState: vi.fn(() => ({
+      proxyMode: 'none',
+      siteLogins: [],
+      globalSpeedLimit: '',
+      perServerConnections: 16,
+      customUserAgent: '',
+      maxAutomaticRetries: 3,
+      mediaCookieSource: 'none',
+    })),
+  }
+}));
+
+describe('useDownloadStore', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    useDownloadStore.setState({
+      downloads: [],
+      backendRegisteredIds: new Set(),
+      pendingOrder: [],
+    });
+  });
+
+  it('Start Queue dispatches exactly once for mixed dispatched/undispatched items', async () => {
+    useDownloadStore.setState({
+      downloads: [
+        { id: '1', url: 'http://test1', fileName: 'f1', status: 'queued', category: 'General', dateAdded: '', queueId: 'MAIN', hasBeenDispatched: true },
+        { id: '2', url: 'http://test2', fileName: 'f2', status: 'queued', category: 'General', dateAdded: '', queueId: 'MAIN', hasBeenDispatched: false },
+      ] as any[],
+      backendRegisteredIds: new Set(['1']), // 1 is already registered, so it skips dispatch
+    });
+
+    vi.mocked(ipc.invokeCommand).mockImplementation(async (cmd: string) => {
+      if (cmd === 'get_pending_order') return ['1', '2'];
+      return undefined;
+    });
+
+    const dispatched = await useDownloadStore.getState().startQueue('MAIN');
+    expect(dispatched).toBe(2); // Both items counted as dispatched/handled
+
+    const calls = vi.mocked(ipc.invokeCommand).mock.calls;
+    const enqueues = calls.filter(c => c[0] === 'enqueue_download');
+    expect(enqueues.length).toBe(1);
+    expect((enqueues[0] as any)[1].item.id).toBe('2');
+  });
+
+  it('resumeDownload unregisters ID and re-dispatches if un-resumable', async () => {
+    useDownloadStore.setState({
+      downloads: [
+        { id: '1', url: 'http://test1', fileName: 'f1', status: 'paused', category: 'General', dateAdded: '', queueId: 'MAIN', hasBeenDispatched: true },
+      ] as any[],
+      backendRegisteredIds: new Set(['1']),
+    });
+
+    vi.mocked(ipc.invokeCommand).mockImplementation(async (cmd: string) => {
+      if (cmd === 'resume_download') return false; // Not resumable
+      if (cmd === 'get_pending_order') return ['1'];
+      return undefined;
+    });
+
+    await useDownloadStore.getState().resumeDownload('1');
+
+    // It should have called resume_download, then unregistered, then enqueue_download
+    const calls = vi.mocked(ipc.invokeCommand).mock.calls;
+    expect(calls.some(c => c[0] === 'resume_download')).toBe(true);
+    expect(calls.some(c => c[0] === 'enqueue_download')).toBe(true);
+    expect(useDownloadStore.getState().backendRegisteredIds.has('1')).toBe(true); // Re-registered by dispatchItem
+  });
+});