Replace single localStorage config with a named-instance store
({ instances: Instance[], activeId }). Adds an instances manager on
the settings page (list / add / edit / delete / activate) and a
sidebar switcher for quick swaps. Existing single-config users are
migrated transparently on first load.
Switching or deleting an instance clears the TanStack Query cache so
data from another deployment never bleeds into the active view.
