Explain React 18's useTransition hook: what problem it solves, how the concurrent scheduler handles non-urgent updates, and how isPending drives UX.

Imagine you're building with LEGOs and your friend asks for a brick while you're in the middle of a big tower. Instead of stopping everything, you say 'just a sec' and finish your tower first. useTransition is like that 'just a sec' for React—it lets important updates (like typing) finish before less important ones (like showing search results) start, and isPending tells you when to show a spinner.