feat(demo): start the demo in the round Fjord Night theme

A visitor without a chosen theme on this device sees the demo in
fjord-night instead of the square default; a chosen theme stays and
real installations are unchanged.

Release v0.27.0.

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

frontend/src/App.tsx

@@ -42,6 +42,8 @@ import {
   AVAILABLE_THEMES,
   customThemeOption,
   initThemeOnBoot,
+  hasChosenTheme,
+  DEMO_THEME,
   watchSystemScheme,
   isCustomThemeId,
   setActiveTheme as applyTheme,
@@ -595,6 +597,13 @@ export const App: React.FC = () => {
     );
   }, [authStatus.demoMode, authStatus.demoResetInSeconds]);
 
+  // Demo: first visit on this device starts with a round theme instead of the square default
+  useEffect(() => {
+    if (!authStatus.demoMode || hasChosenTheme()) return;
+    applyTheme(DEMO_THEME);
+    setActiveThemeState(DEMO_THEME);
+  }, [authStatus.demoMode]);
+
   // Web Share Target: import shared thoughts from external apps
   useEffect(() => {
     if (!user) return;