fix: prevent TaskLane showing stale data when starting new chat

Three race conditions in AssistantChatPage:

1. handleNewChat cleared showTaskLane/activeQuestions/activeActions
   AFTER the createChatSession await — old lane was visible during
   the network call. Moved clears before the await.

2. handleResumeNew never cleared old TaskLane state at all. Added
   upfront clears before the first await.

3. handleSend and handleTaskSubmit had no stale-session guard. If
   the user switched chats while sendChatMessage was in flight, the
   response would set showTaskLane on the wrong session. Added
   sentForChatId snapshot + currentChatRef guard (same pattern
   already used in selectChat).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/AssistantChatPage.tsx

@@ -259,6 +259,11 @@ export default function AssistantChatPage() {
   }, [])
 
   const handleNewChat = async () => {
+    // Clear stale state immediately — don't wait for API to return
+    setShowTaskLane(false)
+    setActiveQuestions([])
+    setActiveActions([])
+    setMessages([])
     try {
       const session = await aiSessionsApi.createChatSession({
         intake_type: 'free_text',
@@ -275,11 +280,6 @@ export default function AssistantChatPage() {
       currentChatRef.current = session.session_id
       setChats(prev => [chatItem, ...prev])
       setActiveChatId(session.session_id)
-      setMessages([])
-      // Clear TaskLane from previous session
-      setShowTaskLane(false)
-      setActiveQuestions([])
-      setActiveActions([])
     } catch {
       toast.error('Failed to create chat')
     }
@@ -315,11 +315,14 @@ export default function AssistantChatPage() {
     setMessages(prev => [...prev, { role: 'user', content: userMessage }])
     setLoading(true)
 
+    const sentForChatId = activeChatId
     try {
       const response = await aiSessionsApi.sendChatMessage(activeChatId, {
         message: userMessage,
         upload_ids: completedUploadIds.length > 0 ? completedUploadIds : undefined,
       })
+      // Guard: discard if user switched to a different chat while this was in flight
+      if (currentChatRef.current !== sentForChatId) return
       analytics.aiFeatureUsed({ feature: 'assistant_chat' })
       setMessages(prev => [
         ...prev,
@@ -327,20 +330,20 @@ export default function AssistantChatPage() {
       ])
       setChats(prev =>
         prev.map(c =>
-          c.id === activeChatId
+          c.id === sentForChatId
             ? { ...c, message_count: c.message_count + 2, title: c.message_count === 0 ? userMessage.slice(0, 100) : c.title, updated_at: new Date().toISOString() }
             : c
         )
       )
       // Load branches if fork was created
-      if (response.fork && activeChatId) {
-        branching.loadBranches(activeChatId)
+      if (response.fork && sentForChatId) {
+        branching.loadBranches(sentForChatId)
       }
       // Show task lane if AI sent questions or actions
       const hasQuestions = response.questions && response.questions.length > 0
       const hasActions = response.actions && response.actions.length > 0
       if (hasQuestions || hasActions) {
-        if (activeChatId) clearTaskState(activeChatId)
+        clearTaskState(sentForChatId)
         setActiveQuestions(response.questions || [])
         setActiveActions(response.actions || [])
         setShowTaskLane(true)
@@ -377,19 +380,22 @@ export default function AssistantChatPage() {
     setMessages(prev => [...prev, { role: 'user', content: userMessage }])
     setLoading(true)
 
+    const sentForChatId = activeChatId
     try {
       const response = await aiSessionsApi.sendChatMessage(activeChatId, { message: userMessage })
+      // Guard: discard if user switched to a different chat while this was in flight
+      if (currentChatRef.current !== sentForChatId) return
       setMessages(prev => [
         ...prev,
         { role: 'assistant', content: response.content, suggestedFlows: response.suggested_flows, fork: response.fork, actions: response.actions, questions: response.questions },
       ])
-      if (response.fork && activeChatId) {
-        branching.loadBranches(activeChatId)
+      if (response.fork && sentForChatId) {
+        branching.loadBranches(sentForChatId)
       }
       // Update task lane based on AI response
       const hasQuestions = response.questions && response.questions.length > 0
       const hasActions = response.actions && response.actions.length > 0
-      if (activeChatId) clearTaskState(activeChatId)
+      clearTaskState(sentForChatId)
       if (hasQuestions || hasActions) {
         setActiveQuestions(response.questions || [])
         setActiveActions(response.actions || [])
@@ -430,6 +436,10 @@ export default function AssistantChatPage() {
   }
 
   const handleResumeNew = async (summary: string) => {
+    // Clear stale state immediately — don't wait for API to return
+    setShowTaskLane(false)
+    setActiveQuestions([])
+    setActiveActions([])
     try {
       const resumePrompt = `I'm continuing a previous troubleshooting session. Here's where we left off:\n\n${summary}\n\nPlease review this context and help me continue from where we stopped.`
       const session = await aiSessionsApi.createChatSession({