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-tauri/src/ipc.rs

@@ -95,6 +95,18 @@ pub struct DownloadItem {
     #[ts(optional)]
     pub media_format_selector: Option<String>,
     pub queue_id: String,
+    #[ts(optional)]
+    pub has_been_dispatched: Option<bool>,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export, export_to = "../../src/bindings/")]
+pub struct EnqueueResult {
+    pub id: String,
+    pub success: bool,
+    #[ts(optional)]
+    pub error: Option<String>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, TS)]