feat: restore active chat session on reload

Save activeChatId to sessionStorage so reloading /assistant restores
the last conversation. Messages load from backend conversation_messages,
task lane restores from sessionStorage (with partial answers intact).

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

frontend/src/pages/AssistantChatPage.tsx

@@ -31,7 +31,10 @@ export default function AssistantChatPage() {
   const navigate = useNavigate()
   const { sessionId: urlSessionId } = useParams<{ sessionId?: string }>()
   const [chats, setChats] = useState<ChatListItem[]>([])
-  const [activeChatId, setActiveChatId] = useState<string | null>(urlSessionId || null)
+  const [activeChatId, setActiveChatId] = useState<string | null>(() => {
+    if (urlSessionId) return urlSessionId
+    try { return sessionStorage.getItem('rf-active-chat-id') } catch { return null }
+  })
   const [messages, setMessages] = useState<MessageWithMeta[]>([])
   const [input, setInput] = useState('')
   const [loading, setLoading] = useState(false)
@@ -77,6 +80,14 @@ export default function AssistantChatPage() {
   const dragCounterRef = useRef(0)
   const prefillHandledRef = useRef(false)
 
+  // Persist active chat ID to sessionStorage
+  useEffect(() => {
+    try {
+      if (activeChatId) sessionStorage.setItem('rf-active-chat-id', activeChatId)
+      else sessionStorage.removeItem('rf-active-chat-id')
+    } catch { /* ignore */ }
+  }, [activeChatId])
+
   // Load chat list from ai_sessions
   useEffect(() => {
     loadChats()
@@ -89,6 +100,13 @@ export default function AssistantChatPage() {
     }
   }, [urlSessionId]) // eslint-disable-line react-hooks/exhaustive-deps
 
+  // Restore session from sessionStorage on mount (when URL has no session ID)
+  useEffect(() => {
+    if (!urlSessionId && activeChatId) {
+      selectChat(activeChatId)
+    }
+  }, []) // eslint-disable-line react-hooks/exhaustive-deps
+
   // Handle prefill from command palette / dashboard handoff
   useEffect(() => {
     const state = location.state as { prefill?: string; uploadIds?: string[] } | null