- Introduced `FrontendAuthProviderExclusive` capability to restrict authentication to a single selected provider. - Added `SetExclusiveProvider` and `ClearExclusiveProvider` methods for managing exclusive providers in the access registry. - Updated `pluginhost` to prioritize and enforce exclusive providers based on plugin priority and ID. - Enhanced RPC capabilities schema to include `FrontendAuthProviderExclusive` field. - Added example plugin and tests for exclusive frontend auth behavior.