Separate ui/, app/, and bwutils/ layers

Activity Browser splits presentation and LCA logic so reusable Qt widgets stay free of app singletons, while Brightway-facing orchestration stays testable and findable. ui/ holds application-agnostic Qt components; app/ holds pages, panes, actions, and dialogs that depend on app.signals / settings / metadata and perform Brightway mutations (especially in actions); bwutils/ holds Brightway/scientific helpers without Qt UI. Place new code with: Brightway-only → bwutils/; no app init needed → ui/; otherwise → app/.