ui: show submit button when the chat input has content (#31324)
ChatFormActions gated the mic and stop buttons on a `canSubmit` prop that
ChatForm never passed, so it was always false and typing text did not swap
them for the submit button. Use `canSend` instead and also hide the
skip-reasoning button while there is content to send.
Assisted-by: Claude (Opus 5.5 Medium)