[FEAT]: Warning popup when duplicate destination URL exists

[steven-tey]

Via a user request:

We have a very distributed, remote, volunteer team, and we're trying to more consistently enforce using shortened links so we have better click tracking, but sometimes team members will go in to create a link for something that already has a link, not knowing that it exists. We'd love to have the option to enable a warning popup if the Destination URL specified when creating a new link already exists in another Short Link.

Implementation: Create a new endpoint that checks if a given destination URL exists in a workspace.

Bonus: Nail down the UI implementation for this – use AnimatedSizeContainer for a smooth transition. See Linear's "similar issues" UX for reference