fix: bump base font_size to 16pt and drop fig.retina for readable README

The old fig.retina=2 at 150 DPI rendered PNGs at 2x resolution that
GitHub markdown can't use, making text appear half its intended size.
Switching to 96 DPI without retina produces images at their natural
display size. Combined with the base font bump (14 → 16pt), titles
and labels now render with proper editorial presence.

man/theme_civilytics_dark.Rd

@@ -5,7 +5,7 @@
 \title{Dark variant of the Civilytics ggplot2 theme}
 \usage{
 theme_civilytics_dark(
-  font_size = 14,
+  font_size = 16,
   font_family = CV_FONT_SANS,
   title_family = CV_FONT_DISPLAY,
   line_size = 0.5,
@@ -21,7 +21,7 @@ theme_civilytics_dark(
 )
 }
 \arguments{
-\item{font_size}{Numeric. Base font size in points. Default `14`.}
+\item{font_size}{Numeric. Base font size in points. Default `16`.}
 
 \item{font_family}{Character. Font family for body/axis text. Default
 `"Inter"` (loaded via showtext).}