Add a regions of interest list to sessions

Goal

Let users save regions to a list in the session and turn a highlight on or off for each one.

Merges the region list idea with the browser's existing highlights. Core already supports these: the Highlight type and highlights state in packages/core/src/browser/state/browserStore.ts, overlays in browser/overlays/Highlights.tsx, a highlight selection mode, and HighlightDialog in packages/ui.
Connect to those rather than rebuilding them. A saved region drives a core highlight.
Keep it simple to start.

Early draft mockup from Sketch, posted as a discussion starter for Friday's feature review. Nothing here is final.

Card layout: the Regions tab, its item menu, and sessions and regions shown side by side as cards.

Open question: how should regions be sorted or grouped? A long flat list gets annoying.