Detect and remember locale

Lingui is currently configured to always default to English when the client loads. Every time a user reloads the site or opens an in-site link in a a new tab, the site will revert to English, ignoring both their browser locale and previous choice.

First, the language toggle button should synchronize a value in local storage, so that the site has memory across sessions and tabs. Second, we should be using Lingui's [locale detection library](https://lingui.dev/ref/locale-detector#usage-with-detect) to check for our local storage key, with the user's browser locale as a fallback, before defaulting to English.

Initially, that locale detection could occur in the `index.tsx`, where we currently always default to English (`i18n.activate('en');`), although this won't address the case where a user has multiple tabs open and switches the language in only one of them. The easiest tab synchronization approach would be a `visibilitychange` event listener (the session store currently does this to refresh session status).