feat: add PRO badge and credits modal for HuggingChat
- Add isPro field to User type and persist from OAuth login
- Fetch fresh isPro status from HuggingFace public API on page load
- Display "Get PRO" button for non-PRO users in sidebar
- Display "PRO" badge for PRO users in sidebar
- Update SubscribeModal to show "Purchase Credits" for PRO users
- Add IconPro component for gradient sparkle icon
