fix: guard all chat response paths against session-switch race condition

handleSend, sendPrefill, and handleResumeNew all make async API calls
that can return after the user has switched to a different session. Without
a guard, the stale response overwrites the new session's questions/actions
state — causing the previous session's FlowPilot Asks to persist.

Fix: capture the session ID before each await and check currentChatRef
after — discarding the response if the user has since switched. This
matches the existing guard pattern in selectChat (lesson #106).

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

frontend/src/pages/AssistantChatPage.tsx

@@ -156,8 +156,10 @@ export default function AssistantChatPage() {
           intake_type: 'free_text',
           intake_content: { text: prefill },
         })
+        const prefillChatId = session.session_id
+        currentChatRef.current = prefillChatId
         const chatItem: ChatListItem = {
-          id: session.session_id,
+          id: prefillChatId,
           title: session.title,
           message_count: 0,
           pinned: false,
@@ -165,28 +167,30 @@ export default function AssistantChatPage() {
           updated_at: new Date().toISOString(),
         }
         setChats(prev => [chatItem, ...prev])
-        setActiveChatId(session.session_id)
+        setActiveChatId(prefillChatId)
         setMessages([{ role: 'user', content: prefill }])
         setLoading(true)
 
-        const response = await aiSessionsApi.sendChatMessage(session.session_id, {
+        const response = await aiSessionsApi.sendChatMessage(prefillChatId, {
           message: prefill,
           upload_ids: uploadIds?.length ? uploadIds : undefined,
         })
+        // Guard: discard if user switched sessions during the API call
+        if (currentChatRef.current !== prefillChatId) return
         setMessages(prev => [
           ...prev,
           { role: 'assistant', content: response.content, suggestedFlows: response.suggested_flows, fork: response.fork, actions: response.actions, questions: response.questions },
         ])
         setChats(prev =>
           prev.map(c =>
-            c.id === session.session_id
+            c.id === prefillChatId
               ? { ...c, message_count: 2, title: prefill.slice(0, 100), updated_at: new Date().toISOString() }
               : c
           )
         )
         // Show task lane if AI sent questions or actions
-        if (response.fork && session.session_id) {
+        if (response.fork && prefillChatId) {
-          branching.loadBranches(session.session_id)
+          branching.loadBranches(prefillChatId)
         }
         const hasQuestions = response.questions && response.questions.length > 0
         const hasActions = response.actions && response.actions.length > 0
@@ -338,6 +342,7 @@ export default function AssistantChatPage() {
   const handleSend = async () => {
     if (!input.trim() || !activeChatId || loading) return
 
+    const sendChatId = activeChatId
     const userMessage = input.trim()
     const completedUploadIds = pendingUploads
       .filter((u) => u.status === 'done' && u.result?.id)
@@ -349,10 +354,13 @@ export default function AssistantChatPage() {
     setLoading(true)
 
     try {
-      const response = await aiSessionsApi.sendChatMessage(activeChatId, {
+      const response = await aiSessionsApi.sendChatMessage(sendChatId, {
         message: userMessage,
         upload_ids: completedUploadIds.length > 0 ? completedUploadIds : undefined,
       })
+      // Guard: if the user switched sessions while the API call was in flight,
+      // discard stale results to prevent overwriting the new session's state
+      if (currentChatRef.current !== sendChatId) return
       analytics.aiFeatureUsed({ feature: 'assistant_chat' })
       setMessages(prev => [
         ...prev,
@@ -360,14 +368,14 @@ export default function AssistantChatPage() {
       ])
       setChats(prev =>
         prev.map(c =>
-          c.id === activeChatId
+          c.id === sendChatId
             ? { ...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) {
+      if (response.fork && sendChatId) {
-        branching.loadBranches(activeChatId)
+        branching.loadBranches(sendChatId)
       }
       // Show task lane if AI sent questions or actions
       const hasQuestions = response.questions && response.questions.length > 0
@@ -380,13 +388,16 @@ export default function AssistantChatPage() {
       // Merge triage update from AI
       if (response.triage_update) mergeTriageUpdate(response.triage_update)
     } catch {
+      if (currentChatRef.current !== sendChatId) return
       setMessages(prev => [
         ...prev,
         { role: 'assistant', content: 'Sorry, something went wrong. Please try again.' },
       ])
     } finally {
-      setLoading(false)
+      if (currentChatRef.current === sendChatId) {
-      requestAnimationFrame(() => inputRef.current?.focus())
+        setLoading(false)
+        requestAnimationFrame(() => inputRef.current?.focus())
+      }
     }
   }
 
@@ -430,21 +441,24 @@ export default function AssistantChatPage() {
       setMessages([{ role: 'user', content: resumePrompt }])
       setLoading(true)
 
-      const response = await aiSessionsApi.sendChatMessage(session.session_id, { message: resumePrompt })
+      const resumeChatId = session.session_id
+      const response = await aiSessionsApi.sendChatMessage(resumeChatId, { message: resumePrompt })
+      // Guard: discard if user switched sessions during the API call
+      if (currentChatRef.current !== resumeChatId) return
       setMessages(prev => [
         ...prev,
         { role: 'assistant', content: response.content, suggestedFlows: response.suggested_flows, fork: response.fork, actions: response.actions, questions: response.questions },
       ])
       setChats(prev =>
         prev.map(c =>
-          c.id === session.session_id
+          c.id === resumeChatId
             ? { ...c, message_count: 2, title: resumePrompt.slice(0, 100), updated_at: new Date().toISOString() }
             : c
         )
       )
       // Show task lane if AI sent questions or actions
-      if (response.fork && session.session_id) {
+      if (response.fork && resumeChatId) {
-        branching.loadBranches(session.session_id)
+        branching.loadBranches(resumeChatId)
       }
       const hasQuestions = response.questions && response.questions.length > 0
       const hasActions = response.actions && response.actions.length > 0