prevent hydration mismatch on branding settings page

• Added a mounted state guard for useTheme() in BrandingSettings.tsx to prevent SSR/client hydration mismatch
• resolvedTheme from next-themes is undefined during SSR but resolves on the client, causing React hydration errors on the branding settings preview section (dark/light conditional rendering)
• The fix ensures SSR and initial hydration both render with isDarkMode = false, then the theme kicks in after mount

Fixes KAZOKUS-FRONTEND-B (9 events, recurring since Feb 21)