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/make_logo_grob.Rd

@@ -4,16 +4,22 @@
 \alias{make_logo_grob}
 \title{Get a Civilytics Logo grob}
 \usage{
-make_logo_grob()
+make_logo_grob(variant = c("light", "dark"))
+}
+\arguments{
+\item{variant}{Character. `"light"` (default) uses the standard dark logo
+suitable for light backgrounds. `"dark"` uses the white logo suitable for
+dark backgrounds (pairs with [theme_civilytics_dark()]).}
 }
 \value{
-a gg object which contains the logo file stored as a Grob suitable for manipulating in
-grid
+A ggplot object (class `"gg"`) containing the logo grob.
 }
 \description{
-Get a Civilytics Logo grob
+Returns a ggplot object containing the Civilytics logo as a rasterGrob,
+ready to compose with plots via [add_logo()] or [add_logo_ga()].
 }
 \examples{
-logo <- make_logo_grob()
-class(logo) # gg
+logo       <- make_logo_grob()           # light background
+logo_dark  <- make_logo_grob("dark")     # dark background
+class(logo)  # "gg" "ggplot"
 }