Add popular fetch progress to Downloads page

- Track active background tasks in an in-memory dict with a lock
- Expose GET /api/channels/tasks returning running task list
- _fetch_popular_task updates done count as each video fetch completes
- Downloads page polls /tasks every 2s and shows progress bars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -64,6 +64,7 @@ export const renameChannelGroup = (id, name) => api.patch(`/channels/groups/${id
 export const addChannelToGroup = (groupId, channelId) => api.post(`/channels/groups/${groupId}/channels/${channelId}`);
 export const removeChannelFromGroup = (groupId, channelId) => api.delete(`/channels/groups/${groupId}/channels/${channelId}`);
 export const bulkChannelAction = (channel_ids, action) => api.post("/channels/bulk-action", { channel_ids, action });
+export const getActiveTasks = () => api.get("/channels/tasks");
 
 // Videos
 export const homeFeed = (page = 0, limit = 25, mode = "ranked", duration = "") =>