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/utils/url.ts

@@ -0,0 +1,24 @@
+export function extractValidDownloadUrls(text: string): string[] {
+  const lines = text.split('\n');
+  const urls: string[] = [];
+  
+  for (const line of lines) {
+    const trimmed = line.trim();
+    if (!trimmed) continue;
+    
+    // Split by whitespace in case multiple URLs are on one line
+    const parts = trimmed.split(/\s+/);
+    for (const part of parts) {
+      try {
+        const url = new URL(part);
+        if (url.protocol === 'http:' || url.protocol === 'https:' || url.protocol === 'ftp:') {
+          urls.push(url.toString());
+        }
+      } catch (e) {
+        // Not a valid URL
+      }
+    }
+  }
+  
+  return [...new Set(urls)];
+}