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.

inst/img/civilytics-mark.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" role="img" aria-label="Civilytics mark">
+  
+  <g fill="none" stroke="#0E1A2B" stroke-linecap="butt">
+    <path d="M 82.77 27.06 A 40 40 0 1 0 82.77 72.94" stroke-width="15"></path>
+    <path d="M 30 50 L 35 50 L 38 44 L 42 56 L 46 42 L 50 58 L 54 50 L 64 50" stroke-width="3.5" stroke-linecap="square"></path>
+  </g>
+</svg>