fix: upload manager — URL imports, active count, board reset

1. URL imports now participate in upload manager pipeline:
   addUrlJob() creates a job row, uploadComplete/setFailed called
   on success/failure — consistent with local file uploads

2. activeCount only counts uploading/processing jobs, not failed.
   Failed jobs no longer inflate the "Uploads (n)" header count.

3. uploadManager.clear() called on boardId change so stale jobs
   from a previous board don't linger in the next board's panel.

frontend/src/pages/Editor.tsx

@@ -64,6 +64,11 @@ export default function Editor({ isPublicView }: EditorProps) {
   const canvasContainerRef = useRef<HTMLDivElement>(null);
   const [uploadManager] = useState(() => new UploadManager());
 
+  // Reset upload manager when switching boards
+  useEffect(() => {
+    uploadManager.clear();
+  }, [boardId, uploadManager]);
+
   // UI state
   const [activeTool, setActiveTool] = useState<ToolType>(ToolType.SELECT);
   const [color, setColor] = useState('#ffffff');