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/lib/theme.ts

@@ -397,6 +397,14 @@ export function getActiveTheme(): ThemeId {
   return 'dark';
 }
 
+/** True once a theme was chosen on this device (the default 'dark' is not stored until then) */
+export function hasChosenTheme(): boolean {
+  return load(THEME_STORAGE_KEY) !== null;
+}
+
+// The demo starts with a round theme, so visitors see the rounded look; a chosen theme stays
+export const DEMO_THEME: ThemeId = 'fjord-night';
+
 // customCss is required for custom themes and ignored for built-in ones
 export function setActiveTheme(themeId: ThemeId, customCss?: string): void {
   const css = isCustomThemeId(themeId) ? customCss ?? null : null;