feat(extensions): add superset.chatbot contribution point (SIP P1.1)

- Add `app` scope and `AppLocation` type to `ViewContributions` manifest schema
- Add host-internal `getViewProvider` and `getRegisteredViewIds` accessors to the views registry
- Add `getActiveChatbot` resolver with first-to-register fallback policy
- Mount `ChatbotMount` in the app shell (fixed bottom-right, persists across routes