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/ipc.ts

@@ -99,6 +99,11 @@ type CommandMap = {
   db_save_queue: { args: { id: string; data: string }; result: void };
   db_delete_queue: { args: { id: string }; result: void };
   create_category_directories: { args: { paths: string[] }; result: void };
+  get_pending_order: { args: undefined; result: string[] };
+  enqueue_download: { args: { item: any }; result: string };
+  enqueue_many: { args: { items: any[] }; result: void };
+  move_in_queue: { args: { id: string; direction: 'Up' | 'Down' | 'Top' | 'Bottom' }; result: string[] };
+  remove_from_queue: { args: { id: string }; result: boolean };
 };
 
 type CommandName = keyof CommandMap;