feat(FN-3336): recover chat isGenerating state on session load to prevent U

Merges FN-3336 to recover chat streaming state on page reload — adds `isGenerating()` and `getGeneratingSessionIds()` to `ChatManager`, enriches the session API to surface active-streaming sessions, and makes `useChat` and `useQuickChat` hooks restore the `isGenerating` flag when loading existing se

Fusion-Task-Id: FN-3336

packages/core/src/chat-types.ts

@@ -63,6 +63,8 @@ export type EnrichedChatSession = ChatSession & {
   lastMessagePreview?: string;
   /** Timestamp of the last message in the session */
   lastMessageAt?: string;
+  /** Whether a generation is currently in progress for this session */
+  isGenerating?: boolean;
 };
 
 /** A parsed @ mention of an agent in a chat message */