feat(tasks): group extension folder downloads (#1594)

ui/flutter/docs/window-capability-rpc.md

@@ -94,6 +94,8 @@ State broadcasting must observe the owning Riverpod state centrally. Do not broa
 - Storage RPC methods expose business capabilities such as create-history operations, not raw `box.get` or `box.put` access.
 - Prefer batch mutations across the window boundary. For example, save all parsed URLs in one `saveCreateHistory(List<String>)` call.
 - New child-window storage requirements must be added to `StorageMethods` and bound in the main registry.
+- Config reads, full config saves, and create-history mutations share one main-window queue. Each history mutation reads, changes, and writes the config within that queue. History reads wait for earlier saves.
+- `StorageMethods` owns `extra.createHistory`; full config saves preserve its latest value inside the queue so stale settings or child-window snapshots cannot overwrite history.
 
 ## 7. Gopeed Backend Migration
 
@@ -128,3 +130,16 @@ Child-window code must not:
 - Parse RPC JSON directly inside a page or widget.
 
 These constraints keep all backend and persistence ownership in the main window and allow multiple child windows to operate concurrently without database locks or duplicated Gopeed runtimes.
+
+## 10. Batch Task Creation
+
+`GopeedMethods.createTaskBatch` carries `CreateTaskBatch` through the existing
+codec registry. Each member accepts either `rid` from resolve or `req`, plus
+creation `opts`. A nonempty `rid` takes priority. BT selections retain their
+original file indexes in `opts.selectFiles`.
+
+Both ordinary and grouped creation return `CreateTaskBatchResult`, an object
+with `taskIds` and an optional `parentId`. A successful response completes the
+batch, including partial success. The preview tracks completed batches so a
+failed batch can be retried without repeating successful batches. Creation does
+not accept a group ID to append members to an existing group.