Faster widget loading and smoother bot setup.

The chat widget appears on your site sooner, and a round of fixes to the new-bot flow removes the dead ends we had seen people hit.

Improved: The widget loads faster: its configuration lookups run in parallel, the bundle is about 12 KB smaller, and the first reply no longer waits on a third-party location lookup.
Improved: Several fixes to the new-bot flow found by watching real setups, including steps that did not advance and errors that were not shown.
Fixed: The Preview button in the bot header is hidden when the Chat Link channel is off, instead of opening a page that did not work.