feat: add map theme variants and fix dark theme text readability

Add theme_civilytics_map(), theme_civilytics_dark_map(), and
theme_civilytics_slide_map() for choropleths — suppress axes, ticks,
gridlines, and axis labels while preserving titles, captions, and legends.

Fix subtitle, caption, and axis text colors in theme_civilytics_dark()
which were inheriting dark ink_2/ink_3 values meant for light backgrounds,
making them hard to read on navy. Now uses navy_200/navy_300 instead.

man/dot-map_theme_extras.Rd

@@ -0,0 +1,16 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/theme.R
+\name{.map_theme_extras}
+\alias{.map_theme_extras}
+\title{Shared map-theme overrides}
+\usage{
+.map_theme_extras()
+}
+\value{
+A partial ggplot2 [ggplot2::theme()] object.
+}
+\description{
+Strips away axes, ticks, gridlines, and axis titles/labels — the elements
+that are meaningless on a choropleth or spatial plot.
+}
+\keyword{internal}