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.Rd

@@ -14,8 +14,10 @@ theme_civilytics(
   rel_large = 16/14,
   ink = unname(civilytics_colors["ink"]),
   paper = unname(civilytics_colors["paper"]),
-  accent = unname(civilytics_colors["accent"]),
-  strip_color = unname(civilytics_colors["paper_2"])
+  accent = unname(civilytics_colors["ember_600"]),
+  strip_color = unname(civilytics_colors["paper_2"]),
+  grid = c("y", "x", "both", "none"),
+  paper_bg = TRUE
 )
 }
 \arguments{
@@ -45,10 +47,18 @@ 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.
@@ -59,6 +69,11 @@ Civilytics brand color palette and typography. Requires ggplot2 >= 4.0.0
 for the `ink`, `paper`, and `accent` base-theme parameters.
 }
 \details{
+Produces an editorial, Pew-style layout: visible x-axis line to ground
+the data, light horizontal gridlines for reference, no panel border or
+y-axis line. Plot title and caption are left-aligned to the full plot
+region.
+
 Brand fonts (Inter for UI text, Libre Franklin for titles) are loaded
 automatically via [showtext] when the package is attached. Call
 [civilytics_load_fonts()] to reload them if needed.
@@ -66,13 +81,21 @@ automatically via [showtext] when the package is attached. Call
 \examples{
 \dontrun{
 library(ggplot2)
+
+# Default editorial theme
 ggplot(mpg, aes(displ, hwy)) +
   geom_point() +
   theme_civilytics()
 
+# With both gridlines and brand colors
 ggplot(mpg, aes(displ, hwy, colour = class)) +
   geom_point() +
   scale_color_civilytics() +
-  theme_civilytics()
+  theme_civilytics(grid = "both")
+
+# Transparent background for embedding
+ggplot(mpg, aes(displ, hwy)) +
+  geom_point() +
+  theme_civilytics(paper_bg = FALSE)
 }
 }