feat: add FlowPilot prefill handoff from command palette to AssistantChatPage

When navigated to /assistant with location.state.prefill, automatically
creates a new chat and sends the prefill message without user interaction.
Clears location state after handling to prevent re-trigger on back navigation.

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

frontend/src/pages/AssistantChatPage.tsx

@@ -1,4 +1,5 @@
 import { useState, useEffect, useRef, useCallback } from 'react'
+import { useLocation, useNavigate } from 'react-router-dom'
 import { Sparkles, Send, Loader2, Flag } from 'lucide-react'
 import { PageMeta } from '@/components/common/PageMeta'
 import { assistantChatApi } from '@/api/assistantChat'
@@ -14,6 +15,8 @@ interface MessageWithMeta extends ChatMessageType {
 }
 
 export default function AssistantChatPage() {
+  const location = useLocation()
+  const navigate = useNavigate()
   const [chats, setChats] = useState<ChatListItem[]>([])
   const [activeChatId, setActiveChatId] = useState<string | null>(null)
   const [messages, setMessages] = useState<MessageWithMeta[]>([])
@@ -22,12 +25,56 @@ export default function AssistantChatPage() {
   const [showConclude, setShowConclude] = useState(false)
   const messagesEndRef = useRef<HTMLDivElement>(null)
   const inputRef = useRef<HTMLTextAreaElement>(null)
+  const prefillHandledRef = useRef(false)
 
   // Load chat list
   useEffect(() => {
     loadChats()
   }, [])
 
+  // Handle prefill from command palette handoff
+  useEffect(() => {
+    const prefill = (location.state as { prefill?: string } | null)?.prefill
+    if (!prefill || prefillHandledRef.current) return
+    prefillHandledRef.current = true
+
+    // Clear the location state so back-navigation doesn't retrigger
+    navigate(location.pathname, { replace: true, state: {} })
+
+    const sendPrefill = async () => {
+      try {
+        const chat = await assistantChatApi.createChat()
+        setChats(prev => [
+          { id: chat.id, title: chat.title, message_count: 0, pinned: false, created_at: chat.created_at, updated_at: chat.updated_at },
+          ...prev,
+        ])
+        setActiveChatId(chat.id)
+        setMessages([{ role: 'user', content: prefill }])
+        setLoading(true)
+
+        const response = await assistantChatApi.sendMessage(chat.id, prefill)
+        setMessages(prev => [
+          ...prev,
+          { role: 'assistant', content: response.content, suggestedFlows: response.suggested_flows },
+        ])
+        setChats(prev =>
+          prev.map(c =>
+            c.id === chat.id
+              ? { ...c, message_count: 2, title: prefill.slice(0, 100), updated_at: new Date().toISOString() }
+              : c
+          )
+        )
+      } catch {
+        toast.error('Failed to start AI conversation')
+      } finally {
+        setLoading(false)
+      }
+    }
+
+    sendPrefill()
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [])
+
   // Auto-scroll
   useEffect(() => {
     messagesEndRef.current?.scrollIntoView({ behavior: 'smooth' })