feat(chat): add useChatStream hook over the stream store

surfsense_web/lib/chat/stream-engine/use-chat-stream.ts

@@ -0,0 +1,18 @@
+"use client";
+
+import { useCallback, useSyncExternalStore } from "react";
+import { chatStreamStore, type ThreadStreamState } from "./store";
+
+/**
+ * Subscribe to the durable streaming state for a given thread.
+ *
+ * Returns the live ``ThreadStreamState`` while a turn is streaming (or
+ * pending re-hydration from the DB), or ``null`` when the thread has no
+ * active overlay — in which case the page falls back to its DB-hydrated
+ * messages. State lives in the module-level {@link chatStreamStore}, so it
+ * survives the chat page unmounting during in-app navigation.
+ */
+export function useChatStream(threadId: number | null): ThreadStreamState | null {
+	const getSnapshot = useCallback(() => chatStreamStore.getSnapshot(threadId), [threadId]);
+	return useSyncExternalStore(chatStreamStore.subscribe, getSnapshot, () => null);
+}