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/lib/theme.ts

@@ -47,15 +47,15 @@ export const AVAILABLE_THEMES: ThemeOption[] = [
     isRounded: false,
   },
   {
-    id: 'high-contrast',
-    name: 'High Contrast (Scharf)',
-    subtitle: 'Tiefschwarz (#000) & weiße Konturen',
-    file: 'src/themes/theme-high-contrast.css',
-    previewBg: '#000000',
-    previewPanel: '#09090b',
-    previewAccent: '#38bdf8',
-    previewText: '#ffffff',
-    isRounded: false,
+    id: 'terracotta',
+    name: 'Terrakotta (Warmes Papier)',
+    subtitle: 'Cremepapier, Tinte & gebrannte Terrakotta (12px)',
+    file: 'src/themes/theme-terracotta.css',
+    previewBg: '#efe6d8',
+    previewPanel: '#fbf6ee',
+    previewAccent: '#c2410c',
+    previewText: '#2b2118',
+    isRounded: true,
   },
   {
     id: 'soft',
@@ -152,7 +152,10 @@ function applyToDocument(themeId: ThemeId, customCss: string | null): void {
   const root = document.documentElement;
   const attr = customCss !== null ? themeAttribute(customCss) : themeId;
   root.setAttribute('data-theme', attr);
-  const light = customCss !== null ? isLightColor(readCssVar(customCss, 'bg-app')) : themeId === 'light';
+  const light =
+    customCss !== null
+      ? isLightColor(readCssVar(customCss, 'bg-app'))
+      : isLightColor(AVAILABLE_THEMES.find((t) => t.id === themeId)?.previewBg);
   root.classList.toggle('dark', !light);
   syncThemeColorMeta();
 }