Add a debug page for comparing offline base-map themes side by side

Renders the bare protomaps-themes-base themes next to the app's actual
offline/online styles (each real app instance, embedded), with pan/zoom
kept in sync via the existing URL-hash mechanism, to make it easy to see
where the hand-tuned offline palette diverges from the online one.

docs/development.adoc

@@ -23,6 +23,17 @@ explicit cache headers, so an edit to `public/` (or `views/`) can
 appear not to have taken effect on the next reload. Force a real
 reload (e.g. Ctrl/Cmd+Shift+R) if that happens.
 
+`/theme_preview.html` is a standalone debug page (not linked from the
+app itself) for comparing offline base-map themes side by side: a bare
+`protomaps-themes-base` theme, an iframe of the real app's offline
+style, and one of its online style, with pan/zoom kept in sync across
+all three via the same URL-hash mechanism `map.js` already uses for
+shareable links. It's a plain static file, so nothing gates it to
+development — it's reachable on a production instance too, same as any
+other file under `public/`. Harmless if it stays that way (it shows
+nothing `/` doesn't already), but worth knowing if that's ever not the
+case.
+
 == Offline map data
 
 To enable the offline style, place a PMTiles file at