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-tauri/Cargo.toml

@@ -23,7 +23,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = ["macos-private-api", "tray-icon", "image-png"] }
+tauri = { version = "2", features = ["macos-private-api", "tray-icon", "image-png", "test"] }
 tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-shell = "2"
@@ -57,6 +57,7 @@ tauri-plugin-store = "2.4.3"
 log = "0.4.32"
 tauri-plugin-log = "2"
 trash = "5"
+async-trait = "0.1"
 [target.'cfg(target_os = "macos")'.dependencies]
 cocoa = "0.25"
 objc = "0.2.7"