fix: task lane no longer self-hides on submit

TaskLane had `if (submitted) return null` which immediately hid the
component after submit, before the AI could respond. Now the parent
controls visibility: the lane stays visible during the AI call, then
either updates with new tasks or clears when the AI sends none.

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

frontend/src/pages/AssistantChatPage.tsx

@@ -310,13 +310,18 @@ export default function AssistantChatPage() {
       if (response.fork && activeChatId) {
         branching.loadBranches(activeChatId)
       }
-      // Show task lane again if AI sends more tasks
+      // Update task lane based on AI response
       const hasQuestions = response.questions && response.questions.length > 0
       const hasActions = response.actions && response.actions.length > 0
       if (hasQuestions || hasActions) {
         setActiveQuestions(response.questions || [])
         setActiveActions(response.actions || [])
         setShowTaskLane(true)
+      } else {
+        // AI sent no new tasks — clear the lane
+        setShowTaskLane(false)
+        setActiveQuestions([])
+        setActiveActions([])
       }
     } catch {
       setMessages(prev => [