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_pal.Rd

@@ -2,28 +2,23 @@
 % Please edit documentation in R/colors.R
 \name{civilytics_pal}
 \alias{civilytics_pal}
-\title{Civilytics brand color palette}
+\title{Civilytics palette function (closure)}
 \usage{
-civilytics_pal(name = "main", n = NULL)
+civilytics_pal(name = "qual", reverse = FALSE)
 }
 \arguments{
-\item{name}{Character. Palette name: `"main"`, `"sequential"`, or
-`"diverging"`. Defaults to `"main"`.}
+\item{name}{Character. Palette name. See `names(civilytics_palettes)`.}
 
-\item{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()].}
+\item{reverse}{Logical. Reverse the palette order. Default `FALSE`.}
 }
 \value{
-A character vector of hex color codes.
+A function that takes integer `n` and returns `n` hex color codes.
 }
 \description{
-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).
+Returns a closure `function(n)` suitable for passing to
+[ggplot2::discrete_scale()] or similar scale constructors.
 }
 \examples{
-civilytics_pal()            # all 6 qualitative colors
-civilytics_pal("sequential", n = 3)
-civilytics_pal("diverging",  n = 5)
+pal_fn <- civilytics_pal("qual")
+pal_fn(4)
 }