Extend the AI Chat widget with three ways to ground a conversation:
- Images: attach images to a message, sent inline as OpenAI multimodal
content parts for vision-capable models. A hint warns when the selected
model likely has no vision support.
- Knowledge bases: pick RAG collections exposed by /api/v1/knowledge/.
- Web pages: ingest a URL via /api/v1/retrieval/process/web.
Open WebUI does not run RAG from the chat-completions `files` parameter,
so attached collections (web pages and knowledge bases) are grounded by
retrieving the most relevant chunks at send time
(/api/v1/retrieval/query/collection) and injecting them as system context.
Also add a Retry action to re-run the last turn after a model failure, and
handle the paginated `{ items, total }` shape of the knowledge endpoint on
newer Open WebUI versions.