feat(cli): make /btw command non-blocking with parallel execution

- Add btwItem state management independent from pendingItem
- Add cancelBtw functionality to abort in-flight BTW API calls
- Allow /btw commands to execute concurrently with main responses
- Add isBtwCommand utility function
- Update BtwMessage UI with cleaner styling (remove spinner)
- Add tests for concurrent /btw execution scenarios
- Update layouts to render BTW messages in fixed bottom area

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/ui/contexts/UIStateContext.tsx

@@ -7,6 +7,7 @@
 import { createContext, useContext } from 'react';
 import type {
   HistoryItem,
+  HistoryItemBtw,
   ThoughtSummary,
   ShellConfirmationRequest,
   ConfirmationRequest,
@@ -101,6 +102,9 @@ export interface UIState {
   staticExtraHeight: number;
   dialogsVisible: boolean;
   pendingHistoryItems: HistoryItemWithoutId[];
+  btwItem: HistoryItemBtw | null;
+  setBtwItem: (item: HistoryItemBtw | null) => void;
+  cancelBtw: () => void;
   nightly: boolean;
   branchName: string | undefined;
   sessionStats: SessionStatsState;