fix(chat): sync currentChatRef when prefill creates a new chat session

The dashboard prefill flow in AssistantChatPage set activeChatId after
creating a new session but never updated currentChatRef.current. Every
later handleSend / handleTaskSubmit then tripped the
`currentChatRef.current !== sentForChatId` guard that was supposed to
discard responses for stale chats — and silently dropped the AI's
follow-up. The user saw their submitted message but no assistant
reply, no toast, no task-lane update.

Mirrors what handleNewChat and handleResumeNew already do. Adds an
e2e regression test that drives the dashboard prefill, submits a
partial task-lane response, and asserts the second AI turn renders.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/AssistantChatPage.tsx

@@ -255,6 +255,12 @@ export default function AssistantChatPage() {
         }
         setChats(prev => [chatItem, ...prev])
         setActiveChatId(session.session_id)
+        // Keep the in-flight guard ref in sync. Without this, currentChatRef
+        // stays at its mount-time value (often a stale id from sessionStorage
+        // or null), so subsequent handleSend / handleTaskSubmit calls bail at
+        // their `currentChatRef.current !== sentForChatId` check and the AI
+        // response is silently dropped.
+        currentChatRef.current = session.session_id
         setMessages([{ role: 'user', content: prefill }])
         setLoading(true)