fix: resolve hydration mismatch error and improve project setup

- Fix React hydration mismatch in ThemeProvider with mounted state check
- Update layout.tsx to use light theme by default instead of system
- Optimize photo filtering with useMemo in page.tsx
- Add Express mock API for development
- Update CLAUDE.md with comprehensive project documentation
- Create backend/ and admin/ directories for future development

frontend/components/theme-provider.tsx

@@ -7,5 +7,15 @@ import {
 } from 'next-themes'
 
 export function ThemeProvider({ children, ...props }: ThemeProviderProps) {
+  const [mounted, setMounted] = React.useState(false)
+
+  React.useEffect(() => {
+    setMounted(true)
+  }, [])
+
+  if (!mounted) {
+    return <>{children}</>
+  }
+
   return <NextThemesProvider {...props}>{children}</NextThemesProvider>
 }