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/src/ipc.rs

@@ -87,9 +87,6 @@ pub struct DownloadItem {
     #[ts(optional)]
     pub media_format_selector: Option<String>,
     pub queue_id: String,
-    #[serde(rename = "_dispatched")]
-    #[ts(optional)]
-    pub dispatched: Option<bool>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, TS)]
@@ -234,3 +231,37 @@ pub struct PersistedSettings {
     pub extension_pairing_token: String,
     pub auto_check_updates: bool,
 }
+
+#[derive(Clone, Copy, Debug, Serialize, Deserialize, TS)]
+#[serde(rename_all = "lowercase")]
+#[ts(export, export_to = "../../src/bindings/")]
+pub enum QueueDirection {
+    Up,
+    Down,
+}
+
+#[derive(Clone, Debug, Serialize, TS)]
+#[ts(export, export_to = "../../src/bindings/")]
+pub struct DownloadStateEvent {
+    pub id: String,
+    pub status: String,
+    pub error: Option<String>,
+}
+
+impl DownloadStateEvent {
+    pub fn new(id: impl Into<String>, status: DownloadStatus) -> Self {
+        Self {
+            id: id.into(),
+            status: status.as_str().to_string(),
+            error: None,
+        }
+    }
+
+    pub fn failed(id: impl Into<String>, error: impl Into<String>) -> Self {
+        Self {
+            id: id.into(),
+            status: DownloadStatus::Failed.as_str().to_string(),
+            error: Some(error.into()),
+        }
+    }
+}