feat(extensions): complete SIP P1 — chatbot mount point & registration

- Add `icon` field to `View` descriptor (static, set at registerView time)
- Track per-extension Disposables in ExtensionsLoader; add deactivateExtension()
- Subscribe ChatbotMount to registry changes so it reacts to activate/deactivate
- Add host-level unhandledrejection isolation to ExtensionsStartup
- Make extension loading non-blocking (host renders immediately, chatbot appears reactively)
- Document superset.chatbot location and chatbot registerView example in public API