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/scale_fill_civilytics.Rd

@@ -4,25 +4,23 @@
 \alias{scale_fill_civilytics}
 \title{Civilytics fill scale for ggplot2}
 \usage{
-scale_fill_civilytics(palette = "main", discrete = TRUE, ...)
+scale_fill_civilytics(palette = "qual", discrete = TRUE, reverse = FALSE, ...)
 }
 \arguments{
-\item{palette}{Character. Palette name passed to [civilytics_pal()].
-Defaults to `"main"`.}
+\item{palette}{Character. Palette name. Defaults to `"qual"`.}
 
-\item{discrete}{Logical. `TRUE` (default) for a discrete scale; `FALSE` for
-a continuous gradient via [ggplot2::scale_fill_gradientn()].}
+\item{discrete}{Logical. `TRUE` (default) for categorical data; `FALSE`
+for a continuous gradient via [ggplot2::scale_fill_gradientn()].}
 
-\item{...}{Additional arguments passed to the underlying ggplot2 scale
-function.}
+\item{reverse}{Logical. Reverse the palette order. Default `FALSE`.}
+
+\item{...}{Additional arguments passed to the ggplot2 scale function.}
 }
 \value{
 A ggplot2 scale object.
 }
 \description{
-Applies a Civilytics brand palette to the `fill` aesthetic. Use
-`discrete = TRUE` for categorical variables and `discrete = FALSE` for
-continuous gradients.
+Applies a Civilytics brand palette to the `fill` aesthetic.
 }
 \examples{
 library(ggplot2)
@@ -32,5 +30,5 @@ ggplot(mpg, aes(class, fill = class)) +
 
 ggplot(faithfuld, aes(waiting, eruptions, fill = density)) +
   geom_tile() +
-  scale_fill_civilytics("sequential", discrete = FALSE)
+  scale_fill_civilytics("seq_ember", discrete = FALSE)
 }