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/add_logo_ga.Rd

@@ -27,6 +27,7 @@ Add a logo to a ggplot2 object
 The resulting object needs to be drawn to the screen using grid.draw()
 }
 \examples{
+\dontrun{
 library(ggplot2); library(grid)
 tmp_plot <- ggplot(mtcars) + aes(x = hp, y = disp) + geom_point() + theme_civilytics()
 tmp_logo <- make_logo_grob()
@@ -34,3 +35,4 @@ plot_and_logo <- add_logo(tmp_plot, tmp_logo)
 grid.draw(plot_and_logo)
 dev.off()
 }
+}