feat: add Open WebUI AI chat integration and widget

Adds a new "openWebUi" integration kind (under a new "ai" category) and an
AI Chat widget for chatting with models served by Open WebUI.

- Integration: Bearer API key auth, model list, native chat history CRUD, and
  token-by-token streaming chat completions (SSE).
- Widget: in-input model picker, streaming responses, and an in-widget history
  panel. Reconstructs conversations from Open WebUI's message tree with a flat
  fallback.
- Streaming is delivered over the WebSocket subscription link.
- Adds integration and widget documentation.

packages/definitions/src/widget.ts

@@ -49,6 +49,7 @@ export const widgetKinds = [
   "beszelAlerts",
   "beszelSystemStats",
   "customApi",
+  "openWebUi",
 ] as const;
 export type WidgetKind = (typeof widgetKinds)[number];