feat: add brand color system, showtext fonts, and dark theme

- R/colors.R: civilytics_colors named vector (from --cv-* CSS vars on
  civilytics.com), three palettes (main/sequential/diverging),
  scale_color_civilytics() and scale_fill_civilytics() for ggplot2
- R/fonts.R: auto-loads Inter + Libre Franklin from Google Fonts via
  showtext in .onLoad(); exports civilytics_load_fonts() for manual retry
- R/theme.R: wires ggplot2 4.0 ink/paper/accent params into theme_grey()
  base, adds title_family + strip_color params; new theme_civilytics_dark()
  using navy_dark background and warm off-white text
- R/logo.R: make_logo_grob(variant) accepts "light"/"dark" to select the
  white logo for dark-background compositions
- DESCRIPTION: adds showtext, sysfonts; pins ggplot2 >= 4.0.0
- tests/testthat/test_theme.R: full test suite for colors, palettes,
  scales, both theme variants, and logo grob variants

man/scale_color_civilytics.Rd

@@ -0,0 +1,36 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/colors.R
+\name{scale_color_civilytics}
+\alias{scale_color_civilytics}
+\title{Civilytics color scale for ggplot2}
+\usage{
+scale_color_civilytics(palette = "main", discrete = TRUE, ...)
+}
+\arguments{
+\item{palette}{Character. Palette name passed to [civilytics_pal()].
+Defaults to `"main"`.}
+
+\item{discrete}{Logical. `TRUE` (default) for a discrete scale; `FALSE` for
+a continuous gradient via [ggplot2::scale_color_gradientn()].}
+
+\item{...}{Additional arguments passed to the underlying ggplot2 scale
+function.}
+}
+\value{
+A ggplot2 scale object.
+}
+\description{
+Applies a Civilytics brand palette to the `colour` aesthetic. Use
+`discrete = TRUE` for categorical variables and `discrete = FALSE` for
+continuous gradients.
+}
+\examples{
+library(ggplot2)
+ggplot(mpg, aes(displ, hwy, colour = class)) +
+  geom_point() +
+  scale_color_civilytics()
+
+ggplot(mpg, aes(displ, hwy, colour = cty)) +
+  geom_point() +
+  scale_color_civilytics("sequential", discrete = FALSE)
+}