feat: wire image uploads into AI assistant chat (vision support)

- Backend: ChatMessageRequest accepts upload_ids, endpoint fetches
  images from S3, base64-encodes them, passes to Claude as multimodal
  content blocks (vision API)
- Backend: add download_file() to storage_service for fetching from S3
- Frontend: handleSend collects completed upload IDs from pendingUploads
  and includes them in the sendChatMessage API call
- Frontend: prefill handler passes upload IDs from dashboard nav state
- Enables paste-screenshot → AI-sees-it flow end-to-end

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

frontend/src/pages/AssistantChatPage.tsx

@@ -55,7 +55,9 @@ export default function AssistantChatPage() {
 
   // Handle prefill from command palette / dashboard handoff
   useEffect(() => {
-    const prefill = (location.state as { prefill?: string } | null)?.prefill
+    const state = location.state as { prefill?: string; uploadIds?: string[] } | null
+    const prefill = state?.prefill
+    const uploadIds = state?.uploadIds
     if (!prefill || prefillHandledRef.current) return
     prefillHandledRef.current = true
 
@@ -80,7 +82,10 @@ export default function AssistantChatPage() {
         setMessages([{ role: 'user', content: prefill }])
         setLoading(true)
 
-        const response = await aiSessionsApi.sendChatMessage(session.session_id, { message: prefill })
+        const response = await aiSessionsApi.sendChatMessage(session.session_id, {
+          message: prefill,
+          upload_ids: uploadIds?.length ? uploadIds : undefined,
+        })
         setMessages(prev => [
           ...prev,
           { role: 'assistant', content: response.content, suggestedFlows: response.suggested_flows },
@@ -183,12 +188,19 @@ export default function AssistantChatPage() {
     if (!input.trim() || !activeChatId || loading) return
 
     const userMessage = input.trim()
+    const completedUploadIds = pendingUploads
+      .filter((u) => u.status === 'done' && u.result?.id)
+      .map((u) => u.result!.id)
     setInput('')
+    setPendingUploads([])
     setMessages(prev => [...prev, { role: 'user', content: userMessage }])
     setLoading(true)
 
     try {
-      const response = await aiSessionsApi.sendChatMessage(activeChatId, { message: userMessage })
+      const response = await aiSessionsApi.sendChatMessage(activeChatId, {
+        message: userMessage,
+        upload_ids: completedUploadIds.length > 0 ? completedUploadIds : undefined,
+      })
       analytics.aiFeatureUsed({ feature: 'assistant_chat' })
       setMessages(prev => [
         ...prev,

frontend/src/types/ai-session.ts

@@ -216,6 +216,7 @@ export interface ChatSessionCreateResponse {
 
 export interface ChatMessageRequest {
   message: string
+  upload_ids?: string[]
 }
 
 export interface ChatMessageResponse {