webui: [a11y] fix keyboard navigation issues in chat interface and sidebar (#23132)

webui: [a11y] fix keyboard navigation issues in chat interface and sidebar (#23132)

* use child snippets for landing and chat message elements

* make ... icon visible in conversation history menu

* conversation history forward tab fix

* add snippet fix for fork icon in conversation history

* focus/keyboard fix for attachment x icon and scroll left/right

* formatting

* fix scroll down issue

* simply Statistics and pointer events in scrolldown

* create storybook tests and move to folder

* improve tests to actually assert on element