feat: add brand color system, showtext fonts, and dark theme

- R/colors.R: civilytics_colors named vector (from --cv-* CSS vars on
  civilytics.com), three palettes (main/sequential/diverging),
  scale_color_civilytics() and scale_fill_civilytics() for ggplot2
- R/fonts.R: auto-loads Inter + Libre Franklin from Google Fonts via
  showtext in .onLoad(); exports civilytics_load_fonts() for manual retry
- R/theme.R: wires ggplot2 4.0 ink/paper/accent params into theme_grey()
  base, adds title_family + strip_color params; new theme_civilytics_dark()
  using navy_dark background and warm off-white text
- R/logo.R: make_logo_grob(variant) accepts "light"/"dark" to select the
  white logo for dark-background compositions
- DESCRIPTION: adds showtext, sysfonts; pins ggplot2 >= 4.0.0
- tests/testthat/test_theme.R: full test suite for colors, palettes,
  scales, both theme variants, and logo grob variants

DESCRIPTION

@@ -14,13 +14,15 @@ BugReports: https://gitea.civilytics.org/Civilytics/civilyticsR/issues
 Depends:
     R (>= 4.1.0)
 Imports:
-    ggplot2,
+    ggplot2 (>= 4.0.0),
     jpeg,
     png,
     stringr,
     gridExtra,
     grid,
-    stringdist
+    stringdist,
+    showtext,
+    sysfonts
 Encoding: UTF-8
 Suggests:
     testthat (>= 3.0.0),