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>

backend/app/schemas/assistant_chat.py

@@ -14,6 +14,7 @@ class ChatCreateRequest(BaseModel):
 
 class ChatMessageRequest(BaseModel):
     message: str = Field(..., min_length=1, max_length=8000)
+    upload_ids: list[UUID] = Field(default_factory=list, max_length=10)
 
 
 class ChatMessageResponse(BaseModel):