fix: prevent task lane from showing previous session's data on switch

Root cause: race condition between setActiveChatId and the persist
effect. When switching from session A to B, setActiveChatId(B) triggers
the persist effect which writes {chatId: B, questions: [A's data]} to
sessionStorage BEFORE the async selectChat clears the task lane. The
sessionStorage fallback then finds chatId === B and restores A's stale
task lane data.

Fix: clear task lane state synchronously in selectChat before the await.
Server-side pending_task_lane restores it if the new session has data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useAssistantSession.ts

@@ -169,7 +169,12 @@ export function useAssistantSession() {
   const selectChat = useCallback(async (chatId: string) => {
     currentChatRef.current = chatId
     setActiveChatId(chatId)
-    // Don't clear task lane yet — wait for server response to decide
+    // Clear task lane immediately to prevent the persist effect from
+    // tagging the old session's data with the new chatId (race condition).
+    // Server-side pending_task_lane will restore it below if it exists.
+    setActiveQuestions([])
+    setActiveActions([])
+    setShowTaskLane(false)
     try {
       const detail = await aiSessionsApi.getSession(chatId)
       if (currentChatRef.current !== chatId) return