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

R/fonts.R

@@ -0,0 +1,51 @@
+# Font family name constants used as defaults in theme functions.
+# These match the --cv-font-* CSS custom properties on www.civilytics.com.
+CV_FONT_DISPLAY <- "Libre Franklin"  # headings / display text
+CV_FONT_SANS    <- "Inter"           # axis text, legends, UI elements
+CV_FONT_SERIF   <- "Source Serif 4"  # body prose (not used in default theme)
+
+# Internal flag so civilytics_load_fonts() is idempotent within a session.
+.cv_fonts_loaded <- FALSE
+
+#' Load Civilytics brand fonts
+#'
+#' Downloads Inter and Libre Franklin from Google Fonts via
+#' [sysfonts::font_add_google()], then calls [showtext::showtext_auto()] so
+#' that all graphics devices render text with those fonts. This is called
+#' automatically when the package loads; use this function to retry if the
+#' initial load failed (e.g., the machine was offline at load time).
+#'
+#' @return Invisibly returns `NULL`.
+#' @export
+#'
+#' @examples
+#' \dontrun{
+#' civilytics_load_fonts()
+#' }
+civilytics_load_fonts <- function() {
+  sysfonts::font_add_google("Inter",          family = "Inter")
+  sysfonts::font_add_google("Libre Franklin", family = "Libre Franklin")
+  showtext::showtext_auto()
+  .cv_fonts_loaded <<- TRUE
+  invisible(NULL)
+}
+
+.onLoad <- function(libname, pkgname) {
+  tryCatch(
+    civilytics_load_fonts(),
+    error = function(e) {
+      # Store the failure so .onAttach can surface a message to the user.
+      options(.civilytics_fonts_failed = TRUE)
+    }
+  )
+}
+
+.onAttach <- function(libname, pkgname) {
+  if (isTRUE(getOption(".civilytics_fonts_failed"))) {
+    packageStartupMessage(
+      "[civilytics] Brand fonts (Inter, Libre Franklin) could not be loaded ",
+      "from Google Fonts. Charts will fall back to system fonts. ",
+      "Call civilytics_load_fonts() once you have an internet connection."
+    )
+  }
+}