Explore infrastructure for multiple interfaces

A/B testing, and "soft launching" ("Click here to try our new beta interface!") both require infrastructure to support multiple UIs at once.

This issue represents identifying a strategy to do this within the context of a browser extension.

The issue names no files, tests, or entry points. Start by mapping how the browser extension currently selects and loads its UI, then investigate strategies for running multiple interfaces concurrently to support A/B testing and soft launches. Done means an agreed, documented infrastructure strategy within the extension context.

Written by the indexing model from the issue text.