feat(queue): implement backend-driven download queue coordinator

This commit replaces the frontend-imperative download dispatcher with a centralized backend `QueueManager`. It acts as the sole concurrency gatekeeper using a single `tokio::sync::Semaphore` across all download paths (aria2 RPC, native HTTP, yt-dlp media).

Backend Changes:
- **queue**: Added `QueueManager` to manage an ordered `VecDeque` of tasks, semaphore permits, and retirement debt (CAS resize).
- **commands**: Replaced direct start commands with `enqueue_download`, `enqueue_many`, `move_in_queue`, and `remove_from_queue`.
- **ipc**: Exported `DownloadStateEvent` and `QueueDirection` to the frontend.
- **tests**: Added 11 integration tests covering idle-parking, idempotent releases, CAS underflow prevention, and gid-completion races.

Frontend Changes:
- **store**: Made `useDownloadStore` reactive to the backend via the `download-state` event.
- **store**: Removed `processQueue` and introduced `pendingOrder` to track the accurate sequence of queued items.
- **ui**: Updated `DownloadItem` with queue visuals (clock icon, position badge).
- **ui**: Added Move Up/Down controls to interact with the backend queue reordering API.

src/components/DownloadTable.tsx

@@ -12,7 +12,7 @@ interface DownloadTableProps {
 }
 
 export const DownloadTable: React.FC<DownloadTableProps> = ({ filter }) => {
-  const { downloads, toggleAddModal, updateDownload, openDeleteModal, redownload } = useDownloadStore();
+  const { downloads, toggleAddModal, openDeleteModal, redownload } = useDownloadStore();
   const { isSidebarVisible, toggleSidebar } = useSettingsStore();
 
   const isMac = navigator.userAgent.includes('Mac');
@@ -93,15 +93,13 @@ export const DownloadTable: React.FC<DownloadTableProps> = ({ filter }) => {
   const handlePause = async (id: string) => {
     try {
       await invoke('pause_download', { id });
-      updateDownload(id, { status: 'paused', speed: '-', eta: '-' });
     } catch (e) {
       console.error("Failed to pause:", e);
     }
   };
 
   const handleResume = (item: DownloadItem) => {
-    updateDownload(item.id, { status: 'queued', _dispatched: false, speed: '-', eta: '-' });
-    useDownloadStore.getState().processQueue();
+    useDownloadStore.getState().resumeDownload(item.id);
   };
 
   const handleDelete = (id: string) => {