The logo upload currently uses browser native alert() popup for validation errors. This feels inconsistent with the rest of the application which uses modern Toasts.

Can we switch these alerts to use toast.error()? This would ensure the error feedback matches the design system used everywhere else in the platform. Maybe a red Toast notification should appear explaining the error, matching other form validations.

Steps to Reproduce

1. Go to Settings → Appearance
2. Upload a JPEG file (wrong format)
3. Observe the native browser alert window