feat(downloads): live speeds and active/completed split on the download panel (DL-2)

- GET /download-tasks/board?state=active|completed: per-torrent dl/up speed,
  ETA, seeds/leechs, downloader total speed (transfer/info), counts for both
  groups, latest organize result (status, reason code, open decision)
- live qB data cached per instance for 2 s with in-flight coalescing
- panel: total speed line, 进行中 / 已完成 segments remembered locally,
  4 s polling while the page is visible, mobile layout, "—" for zero speed

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/index.ts

@@ -9,6 +9,7 @@ export * from "./capabilities-io.ts";
 export * from "./capability.ts";
 export * from "./common.ts";
 export * from "./content.ts";
+export * from "./download-board.ts";
 export * from "./events.ts";
 export * from "./execution.ts";
 export * from "./identity.ts";