test(ui): replace window.__map test-only global with a non-global affordance

[comnam90]

Spun out of code review on #103 (item #4).

Context

layouts/index.html assigns window.__map = map so the two NZ regression tests in tests/ui.spec.ts can call m.setView(...) and m.latLngToContainerPoint(...) directly. This works but bleeds test affordance into production:

// layouts/index.html (around L1158)
window.__map = map;

Proposed fix

Either:

• Gate behind a build-time flag: if (window.__TEST__) window.__map = map; and have Playwright set window.__TEST__ = true before page load.
• Or expose the map via a data-leaflet-map="ready" attribute and have tests resolve it through Leaflet's own _leaflet_id registry.

Acceptance

• window.__map no longer set in production build
• NZ popup-fit regression tests still pass on Mobile Chrome and Mobile Safari