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/civilytics_colors.Rd

@@ -0,0 +1,19 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/colors.R
+\name{civilytics_colors}
+\alias{civilytics_colors}
+\title{Civilytics brand colors}
+\format{
+A named character vector of hex color codes.
+}
+\usage{
+civilytics_colors
+}
+\description{
+A named character vector of all Civilytics brand colors, matching the CSS
+custom properties defined on www.civilytics.com (`--cv-*` variables).
+}
+\examples{
+civilytics_colors["accent"]
+civilytics_colors[c("ink", "paper")]
+}