feat: expand color system to 53 colors/10 palettes, add grid/slide themes, and README gallery

Replace the 16-color / 3-palette system with the full Civilytics design
system: 53 named colors (full navy, ember, violet ramps plus teal, plum,
moss, brass supporting hues) and 10 visualization palettes (qual,
qual_warm, qual_cool, seq_ember, seq_navy, seq_violet, seq_paper_ink,
div_navy_ember, div_violet_ember).

Enrich theme_civilytics() with grid and paper_bg parameters, add
theme_civilytics_slide() for presentations, and load all four brand fonts
(Inter, Libre Franklin, Source Serif 4, JetBrains Mono).

Add README.Rmd with rendered gallery showing all palettes, theme
variants, grid options, dark/slide modes, facets, and scale usage.

man/theme_civilytics_dark.Rd

@@ -13,9 +13,11 @@ theme_civilytics_dark(
   rel_tiny = 11/14,
   rel_large = 16/14,
   ink = unname(civilytics_colors["paper"]),
-  paper = unname(civilytics_colors["navy_dark"]),
-  accent = unname(civilytics_colors["accent_dark"]),
-  strip_color = unname(civilytics_colors["navy"])
+  paper = unname(civilytics_colors["navy_700"]),
+  accent = unname(civilytics_colors["ember_400"]),
+  strip_color = unname(civilytics_colors["navy_600"]),
+  grid = c("y", "x", "both", "none"),
+  paper_bg = TRUE
 )
 }
 \arguments{
@@ -45,31 +47,33 @@ Default `11/14`.}
 [civilytics_colors]`["paper"]` (`#FAF7F2`).}
 
 \item{accent}{Character. Hex code for accent/highlight color. Defaults to
-[civilytics_colors]`["accent"]` (`#C25311`).}
+[civilytics_colors]`["ember_600"]` (`#C25311`).}
 
 \item{strip_color}{Character. Hex code for facet strip background. Defaults
 to [civilytics_colors]`["paper_2"]` (`#F2EDE4`).}
+
+\item{grid}{Character. Which major gridlines to draw: `"y"` (default,
+horizontal only), `"x"` (vertical only), `"both"`, or `"none"`.}
+
+\item{paper_bg}{Logical. If `TRUE` (default), fill the plot and panel
+backgrounds with the warm `paper` color. Set to `FALSE` for a
+transparent background (useful for slides or overlay on colored
+surfaces).}
 }
 \value{
 A complete ggplot2 [ggplot2::theme()] object.
 }
 \description{
-A convenience wrapper around [theme_civilytics()] with dark-background
-defaults: navy (`#1A2E4A`) background, warm off-white (`#FAF7F2`) text, and
-a lighter orange accent (`#E07840`) suitable for dark backgrounds. Pair with
-`make_logo_grob(variant = "dark")` to use the white logo.
+Convenience wrapper around [theme_civilytics()] with dark-background
+defaults: navy (`#1A2E4A`) paper, warm off-white (`#FAF7F2`) ink, and
+a lighter ember accent (`#E07840`). Facet strips use primary navy.
 }
 \details{
-All parameters from [theme_civilytics()] are accepted and override the dark
-defaults.
+Pair with `make_logo_grob(variant = "dark")` for the white logo.
 }
 \examples{
 \dontrun{
 library(ggplot2)
-ggplot(mpg, aes(displ, hwy)) +
-  geom_point() +
-  theme_civilytics_dark()
-
 ggplot(mpg, aes(displ, hwy, colour = class)) +
   geom_point() +
   scale_color_civilytics() +