Style Elements chat with a new custom CSS escape hatch

Embedders get a supported way to restyle chat beyond the built-in theme:

injects extra CSS into the shadow root after the built-in stylesheet, reaching the stable

class hooks that host-page CSS can't touch across the shadow DOM boundary.

Features

• Add
  to
  #3388 - Pass extra CSS to be injected into the Elements shadow root after the built-in stylesheet, giving embedders a supported way to restyle the stable
  class hooks. (Author: @adaam2)

Bug fixes

• Fix dropped or duplicated first message on a cold page load #3388 - The history-enabled chat runtime now mounts immediately instead of waiting for auth to resolve, so the first message sent right after a cold load is no longer lost or sent twice. The custom transport is also resolved independently of auth and connection-status churn, so it no longer rebuilds mid-turn and discards an in-flight optimistic message. (Author: @adaam2)