Update default theme to light and fix data source subtitle

Default to light theme instead of system preference, and update
dashboard subtitle to reflect VivaTicket as the data source.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -46,9 +46,9 @@ function App() {
   const [dataSource, setDataSource] = useState<string>('museums');
   const [theme, setTheme] = useState<string>(() => {
     if (typeof window !== 'undefined') {
-      return localStorage.getItem('hihala_theme') || 'system';
+      return localStorage.getItem('hihala_theme') || 'light';
     }
-    return 'system';
+    return 'light';
   });
 
   useEffect(() => {