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_slide.Rd

@@ -11,7 +11,7 @@ theme_civilytics_slide(
   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`).}