feat(themes): Terrakotta replaces High Contrast; equal header buttons on phones

chore(release): v0.12.0

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -11,7 +11,7 @@
 
 @import './themes/theme-dark.css';
 @import './themes/theme-light.css';
-@import './themes/theme-high-contrast.css';
+@import './themes/theme-terracotta.css';
 @import './themes/theme-soft.css';
 @import './themes/theme-polarized.css';
 @import './themes/theme-suri.css';
@@ -426,7 +426,7 @@ body {
     --code-variable: #9cdcfe;
   }
 
-  :root:not(.dark),
+  :root:not(.dark):not([data-theme="terracotta"]),
   :root[data-theme="light"] {
     --code-keyword: #0000ff;
     --code-string: #a31515;