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/types/ai-session.ts

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