Detect title collisions and surface them in the config UI as non-blocking alerts.

For every configured opt-in entry, the CommunityConfiguration UI shows whether the chosen local title currently collides with an existing local article on the wiki. The check is deliberately non-blocking rather than a hard validation error because collisions can arise either at opt-in time (the sysop picked a title that already exists) or subsequently (the local article was created or moved into place after the opt-in), and the correct response in both cases is the same: show the sysop that the opt-in is currently not surfacing at its configured title, and let them decide whether to change the title, remove the opt-in, or leave it in place on the expectation that the local article may itself be removed later. The collision check runs against the local page table (or the equivalent title-lookup API) at config-render time, so the alert reflects the wiki's current state rather than whatever was true when the config was last edited.