Capability 5 — the app-shell. One element,
<doxa-maps-spa>, is a vertical-tab layout of the whole DX-maps app:
tab 1 = Big Picture (its real <bp-big-picture>
header + people-groups grid, mounted from the built big-picture.js), and the
other tabs are every production research map, each nested as its own full SPA. Switch tabs
with no page reload; each map tab loads lazily on first open.