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/colors.R

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+#' Civilytics brand colors
+#'
+#' A named character vector of all Civilytics brand colors, matching the CSS
+#' custom properties defined on www.civilytics.com (`--cv-*` variables).
+#'
+#' @format A named character vector of hex color codes.
+#' @export
+#'
+#' @examples
+#' civilytics_colors["accent"]
+#' civilytics_colors[c("ink", "paper")]
+civilytics_colors <- c(
+  # Paper (backgrounds) — warm off-white scale
+  paper       = "#FAF7F2",
+  paper_2     = "#F2EDE4",
+  paper_3     = "#E6DFD1",
+  # Rules (borders/dividers)
+  rule        = "#D6CEBD",
+  rule_strong = "#B8AE97",
+  # Ink (text / foreground) — dark-to-light navy-grey scale
+  ink         = "#0E1A2B",
+  ink_2       = "#2B3A52",
+  ink_3       = "#5A6A82",
+  ink_4       = "#8C97AB",
+  # Navy — primary brand blue
+  navy        = "#22406A",
+  navy_dark   = "#1A2E4A",
+  # Accent — burnt orange
+  accent      = "#C25311",
+  accent_dark = "#E07840",
+  accent_50   = "#FDF1E4",
+  accent_100  = "#FBE0C6",
+  accent_200  = "#F8C8A3"
+)
+
+# Internal named list of visualization palettes.
+# Each entry is a character vector of hex codes ordered for visual distinction.
+.cv_palettes <- list(
+  # Qualitative: distinct hues for categorical data (up to 6 categories)
+  main = unname(civilytics_colors[c(
+    "navy_dark", "accent", "ink_3", "rule_strong", "navy", "accent_200"
+  )]),
+  # Sequential: light-to-dark navy for ordered/continuous data
+  sequential = unname(civilytics_colors[c(
+    "paper", "ink_4", "ink_3", "navy", "navy_dark"
+  )]),
+  # Diverging: orange <-> neutral <-> navy for data with a meaningful midpoint
+  diverging = unname(civilytics_colors[c(
+    "accent", "accent_200", "paper", "ink_4", "navy_dark"
+  )])
+)
+
+# Internal palette generator: returns a function(n) -> character vector.
+.cv_pal_fun <- function(name) {
+  pal <- .cv_palettes[[name]]
+  if (is.null(pal)) {
+    stop(sprintf(
+      "'%s' is not a valid Civilytics palette. Choose from: %s",
+      name, paste(names(.cv_palettes), collapse = ", ")
+    ), call. = FALSE)
+  }
+  function(n) {
+    if (n > length(pal)) grDevices::colorRampPalette(pal)(n) else pal[seq_len(n)]
+  }
+}
+
+#' Civilytics brand color palette
+#'
+#' Returns a character vector of hex codes from a named Civilytics palette.
+#' Available palettes: `"main"` (qualitative, up to 6), `"sequential"` (light
+#' to dark navy), `"diverging"` (orange–neutral–navy).
+#'
+#' @param name Character. Palette name: `"main"`, `"sequential"`, or
+#'   `"diverging"`. Defaults to `"main"`.
+#' @param n Integer or `NULL`. Number of colors to return. If `NULL`, returns
+#'   all colors in the palette. If `n` exceeds the number of defined stops,
+#'   colors are interpolated via [grDevices::colorRampPalette()].
+#'
+#' @return A character vector of hex color codes.
+#' @export
+#'
+#' @examples
+#' civilytics_pal()            # all 6 qualitative colors
+#' civilytics_pal("sequential", n = 3)
+#' civilytics_pal("diverging",  n = 5)
+civilytics_pal <- function(name = "main", n = NULL) {
+  f <- .cv_pal_fun(name)
+  len <- length(.cv_palettes[[name]])
+  f(if (is.null(n)) len else n)
+}
+
+#' Civilytics color scale for ggplot2
+#'
+#' Applies a Civilytics brand palette to the `colour` aesthetic. Use
+#' `discrete = TRUE` for categorical variables and `discrete = FALSE` for
+#' continuous gradients.
+#'
+#' @param palette Character. Palette name passed to [civilytics_pal()].
+#'   Defaults to `"main"`.
+#' @param discrete Logical. `TRUE` (default) for a discrete scale; `FALSE` for
+#'   a continuous gradient via [ggplot2::scale_color_gradientn()].
+#' @param ... Additional arguments passed to the underlying ggplot2 scale
+#'   function.
+#'
+#' @return A ggplot2 scale object.
+#' @export
+#'
+#' @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)
+scale_color_civilytics <- function(palette = "main", discrete = TRUE, ...) {
+  if (discrete) {
+    ggplot2::discrete_scale("colour", palette = .cv_pal_fun(palette), ...)
+  } else {
+    ggplot2::scale_color_gradientn(
+      colours = grDevices::colorRampPalette(.cv_palettes[[palette]])(256),
+      ...
+    )
+  }
+}
+
+#' Civilytics fill scale for ggplot2
+#'
+#' Applies a Civilytics brand palette to the `fill` aesthetic. Use
+#' `discrete = TRUE` for categorical variables and `discrete = FALSE` for
+#' continuous gradients.
+#'
+#' @param palette Character. Palette name passed to [civilytics_pal()].
+#'   Defaults to `"main"`.
+#' @param discrete Logical. `TRUE` (default) for a discrete scale; `FALSE` for
+#'   a continuous gradient via [ggplot2::scale_fill_gradientn()].
+#' @param ... Additional arguments passed to the underlying ggplot2 scale
+#'   function.
+#'
+#' @return A ggplot2 scale object.
+#' @export
+#'
+#' @examples
+#' library(ggplot2)
+#' ggplot(mpg, aes(class, fill = class)) +
+#'   geom_bar() +
+#'   scale_fill_civilytics()
+#'
+#' ggplot(faithfuld, aes(waiting, eruptions, fill = density)) +
+#'   geom_tile() +
+#'   scale_fill_civilytics("sequential", discrete = FALSE)
+scale_fill_civilytics <- function(palette = "main", discrete = TRUE, ...) {
+  if (discrete) {
+    ggplot2::discrete_scale("fill", palette = .cv_pal_fun(palette), ...)
+  } else {
+    ggplot2::scale_fill_gradientn(
+      colours = grDevices::colorRampPalette(.cv_palettes[[palette]])(256),
+      ...
+    )
+  }
+}