fade in prompt metadata transitions

[thdxr]

Summary

• fade the prompt agent, model, provider, and variant labels in when their metadata row becomes visible instead of snapping in
• extract the timer-driven fade-in signal into tui/util/signal.ts so prompt rendering can reuse the same animation primitive cleanly
• respect animations_enabled and keep the existing typecheck passing with the unrelated local sync.tsx edit left out of the commit

Testing

• bun typecheck

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.