feat: bump title sizes, add font_scale to logo functions, ship SVGs

- Change rel_large default from 16/14 to 20/14 (~1.43x) to match the
  Civilytics editorial design system. Subtitle now renders at 1.0x base
  instead of 0.86x.
- Switch axis.text to rel() sizing so all text elements cascade from the
  root font_size, enabling uniform scaling.
- Add font_scale parameter (default 1.1) to civilytics_logo(),
  add_logo(), and add_logo_ga() — inflates text by ~10% before
  arrangeGrob composition to compensate for viewport shrinkage.
- Ship SVG logo variants (mark, wordmark, pulse) in inst/img/ for
  Quarto/HTML templates.
- Add "Maintaining brand assets" section to README documenting inst/img/
  file inventory and update workflow.
- Document full font size hierarchy in theme_civilytics() roxygen.

man/theme_civilytics.Rd

@@ -11,7 +11,7 @@ theme_civilytics(
   line_size = 0.5,
   rel_small = 12/14,
   rel_tiny = 11/14,
-  rel_large = 16/14,
+  rel_large = 20/14,
   ink = unname(civilytics_colors["ink"]),
   paper = unname(civilytics_colors["paper"]),
   accent = unname(civilytics_colors["ember_600"]),
@@ -38,7 +38,8 @@ Default `"Libre Franklin"` (loaded via showtext).}
 Default `11/14`.}
 
 \item{rel_large}{Numeric. Scale factor for large text (titles) relative to
-`font_size`. Default `16/14`.}
+`font_size`. Default `20/14` (~1.43x), matching the Civilytics editorial
+design system.}
 
 \item{ink}{Character. Hex code for foreground/text color. Defaults to
 [civilytics_colors]`["ink"]` (`#0E1A2B`).}
@@ -78,6 +79,28 @@ 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.
 }
+\section{Font size hierarchy}{
+
+All text sizes are derived from `font_size` using relative scale factors.
+At the default `font_size = 14`:
+
+| Element | Scale factor | Default size |
+|:--------|:-------------|:-------------|
+| Plot title | `rel_large` (1.43x) | ~20 pt |
+| Subtitle | 1.0x | 14 pt |
+| Axis text (tick labels) | `rel_small` (0.86x) | ~12 pt |
+| Axis titles | `rel_small` (0.86x) | ~12 pt |
+| Legend text | `rel_small` (0.86x) | ~12 pt |
+| Caption | `rel_tiny` (0.79x) | ~11 pt |
+| Legend title | `rel_tiny` (0.79x) | ~11 pt |
+| Strip text (facets) | `rel_small` (0.86x) | ~12 pt |
+
+To uniformly scale all text, change `font_size`. To adjust only the title
+prominence, change `rel_large`. When using [civilytics_logo()] to add a
+logo below the plot, pass `font_scale` to compensate for viewport
+shrinkage.
+}
+
 \examples{
 \dontrun{
 library(ggplot2)
@@ -97,5 +120,10 @@ ggplot(mpg, aes(displ, hwy, colour = class)) +
 ggplot(mpg, aes(displ, hwy)) +
   geom_point() +
   theme_civilytics(paper_bg = FALSE)
+
+# Larger text for poster or display
+ggplot(mpg, aes(displ, hwy)) +
+  geom_point() +
+  theme_civilytics(font_size = 18)
 }
 }