feat: persist task lane across submits and session reloads

Task lane questions/actions are now saved to a pending_task_lane JSONB
column on ai_sessions, restoring them on session switch or page reload.
Partial submit no longer force-clears the lane — the AI response
controls what stays. Also removes redundant "New Session" button from
the sidebar (dashboard already provides this).

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

frontend/src/pages/AssistantChatPage.tsx

@@ -160,7 +160,7 @@ export default function AssistantChatPage() {
 
   const selectChat = useCallback(async (chatId: string) => {
     setActiveChatId(chatId)
-    // Clear TaskLane when switching chats
+    // Clear TaskLane when switching chats — will restore from backend if available
     setShowTaskLane(false)
     setActiveQuestions([])
     setActiveActions([])
@@ -172,6 +172,16 @@ export default function AssistantChatPage() {
           content: m.content,
         }))
       )
+      // Restore task lane from persisted state
+      if (detail.pending_task_lane) {
+        const q = detail.pending_task_lane.questions || []
+        const a = detail.pending_task_lane.actions || []
+        if (q.length > 0 || a.length > 0) {
+          setActiveQuestions(q)
+          setActiveActions(a)
+          setShowTaskLane(true)
+        }
+      }
     } catch {
       setMessages([])
     }
@@ -288,11 +298,6 @@ export default function AssistantChatPage() {
     }
     const userMessage = parts.join('\n\n')
 
-    // Close the task lane
-    setShowTaskLane(false)
-    setActiveQuestions([])
-    setActiveActions([])
-
     setMessages(prev => [...prev, { role: 'user', content: userMessage }])
     setLoading(true)