fix(streamdown): increase dropdown z-index (#463)

* fix(streamdown): increase dropdown z-index

* chore: rm duplicate class

* chore: increase dropdown z-index

* add patch changeset for dropdown z-index fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Hayden Bleasel <hello@haydenbleasel.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>