Isolate webchat message scrolling from host scroll hijacking

[pelcasandra]

Summary

• Added scroll isolation for the webchat message list so host-page smooth-scroll code does not block conversation scrolling.
• Applied overscroll-behavior, touch-action, and Lenis prevention attributes on the messages container.
• Stopped wheel and touchmove propagation from inside the message scroller and cleaned those listeners up on disconnect.
• Added tests covering the new isolation behavior and teardown.

Testing

• yarn test __tests__/controllers/webchat_controller_test.js
• yarn build
• yarn test

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Note

Low Risk
Scoped to the messages container scroll UX and event propagation; no auth, API, or data-path changes.

Overview
This PR keeps webchat conversation scrolling working when the host page uses smooth-scroll libraries (e.g. Lenis) that would otherwise capture wheel/touch gestures.

On connect, the messages container gets scroll-containment styles (overscroll-behavior, touch-action, iOS momentum scrolling) and Lenis prevent data attributes. Capture-phase, passive wheel and touchmove listeners call stopHostScrollPropagation so those events do not bubble to the host. Disconnect removes those listeners alongside existing teardown.

The diff adds Jest coverage for setup, propagation blocking, and listener cleanup in webchat_controller_test.js.

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