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.

R/fonts.R

@@ -2,7 +2,8 @@
 # These match the --cv-font-* CSS custom properties on www.civilytics.com.
 CV_FONT_DISPLAY <- "Libre Franklin"  # headings / display text
 CV_FONT_SANS    <- "Inter"           # axis text, legends, UI elements
-CV_FONT_SERIF   <- "Source Serif 4"  # body prose (not used in default theme)
+CV_FONT_SERIF   <- "Source Serif 4"  # body prose / editorial long-form
+CV_FONT_MONO    <- "JetBrains Mono"  # code, data tables, numeric callouts
 
 # Internal flag so civilytics_load_fonts() is idempotent within a session.
 .cv_fonts_loaded <- FALSE
@@ -25,6 +26,8 @@ CV_FONT_SERIF   <- "Source Serif 4"  # body prose (not used in default theme)
 civilytics_load_fonts <- function() {
   sysfonts::font_add_google("Inter",          family = "Inter")
   sysfonts::font_add_google("Libre Franklin", family = "Libre Franklin")
+  sysfonts::font_add_google("Source Serif 4", family = "Source Serif 4")
+  sysfonts::font_add_google("JetBrains Mono", family = "JetBrains Mono")
   showtext::showtext_auto()
   .cv_fonts_loaded <<- TRUE
   invisible(NULL)
@@ -43,8 +46,9 @@ civilytics_load_fonts <- function() {
 .onAttach <- function(libname, pkgname) {
   if (isTRUE(getOption(".civilytics_fonts_failed"))) {
     packageStartupMessage(
-      "[civilytics] Brand fonts (Inter, Libre Franklin) could not be loaded ",
-      "from Google Fonts. Charts will fall back to system fonts. ",
+      "[civilytics] Brand fonts (Inter, Libre Franklin, Source Serif 4, ",
+      "JetBrains Mono) could not be loaded from Google Fonts. ",
+      "Charts will fall back to system fonts. ",
       "Call civilytics_load_fonts() once you have an internet connection."
     )
   }