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/civilytics_colors.Rd

@@ -12,10 +12,12 @@ civilytics_colors
 }
 \description{
 A named character vector of all Civilytics brand colors, matching the CSS
-custom properties defined on www.civilytics.com (`--cv-*` variables).
+custom properties in the Civilytics design system (`--cv-*` variables).
+Includes full ramps for navy, ember, and violet, plus supporting hues
+(teal, plum, moss, brass) and semantic status colors.
 }
 \examples{
-civilytics_colors["accent"]
-civilytics_colors[c("ink", "paper")]
+civilytics_colors["ink"]
+civilytics_colors[c("navy_600", "ember_600", "teal_600")]
 }
 \keyword{datasets}