feat(dashboard): add performance and specifications tabs (#1816)

* feat(dashboard): add performance and specifications tabs

Replace the grouped Performance sidebar destination with one Dashboard containing peer Performance and System Specifications tabs. Preserve Classic Navigation, migrate legacy grouped targets after settings load, and unmount inactive live content.

Record the corrected navigation hierarchy in ADR 0010, product vocabulary, and project guidance after the maintainer clarification.

* fix(dashboard): isolate startup and specification state

Keep the selected display unset until Application Preferences load so legacy targets never render under the wrong navigation layout. Store System Specifications ordering and visibility separately from Classic Dashboard preferences.