The external guest link icon was fading in/out on every WebSocket update,
creating a distracting visual effect. Now the fade-in animation only plays
when the URL first becomes available (initial load or when saving a new URL),
rather than re-triggering on every state update.
- Add fadeIn keyframe animation to Tailwind config
- Track animation state with shouldAnimateIcon signal
- Only animate on transition from no URL to having a URL
- Clear animation flag after 200ms to prevent re-triggering
