fix: resolve locked namespace binding in civilytics_load_fonts() and default theme to transparent background

- Replace .cv_fonts_loaded <<- TRUE with environment-based state
  (.cv_state) to avoid 'locked namespace binding' error (#18)
- Add force=FALSE argument for idempotent font loading with guard check
- Flip theme_civilytics() paper_bg default to FALSE (transparent) (#7)
- Update roxygen docs and examples for both changes

R/theme.R

@@ -36,10 +36,12 @@
 #'   to [civilytics_colors]`["paper_2"]` (`#F2EDE4`).
 #' @param grid Character. Which major gridlines to draw: `"y"` (default,
 #'   horizontal only), `"x"` (vertical only), `"both"`, or `"none"`.
-#' @param 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).
+#' @param paper_bg Logical. If `TRUE`, fill the plot and panel backgrounds
+#'   with the warm `paper` color (Civilytics cream). Default is `FALSE`
+#'   (transparent) so that figures composite cleanly onto any background.
+#'   Set to `TRUE` for the branded cream canvas. Note: a transparent device
+#'   background (e.g., `dev = "ragg_png"`, `dev.args = list(background =
+#'   "transparent")`) is also needed for fully-transparent PNG exports.
 #'
 #' @section Font size hierarchy:
 #' All text sizes are derived from `font_size` using relative scale factors.
@@ -68,7 +70,7 @@
 #' \dontrun{
 #' library(ggplot2)
 #'
-#' # Default editorial theme
+#' # Default — transparent background for embedding
 #' ggplot(mpg, aes(displ, hwy)) +
 #'   geom_point() +
 #'   theme_civilytics()
@@ -79,10 +81,10 @@
 #'   scale_color_civilytics() +
 #'   theme_civilytics(grid = "both")
 #'
-#' # Transparent background for embedding
+#' # Branded cream background (opt-in)
 #' ggplot(mpg, aes(displ, hwy)) +
 #'   geom_point() +
-#'   theme_civilytics(paper_bg = FALSE)
+#'   theme_civilytics(paper_bg = TRUE)
 #'
 #' # Larger text for poster or display
 #' ggplot(mpg, aes(displ, hwy)) +
@@ -102,7 +104,7 @@ theme_civilytics <- function(
     accent       = unname(civilytics_colors["ember_600"]),
     strip_color  = unname(civilytics_colors["paper_2"]),
     grid         = c("y", "x", "both", "none"),
-    paper_bg     = TRUE) {
+    paper_bg     = FALSE) {
 
   grid <- match.arg(grid)
   half_line  <- font_size / 2