refactor(ui): remove unused props/imports and delegate project path selection to tabs\n\n- Remove unused icons/components and dead handlers (e.g., directory selection) across UI components\n- Drop unused callbacks from props (onBack, onMCPClick, onUsageClick, etc.)\n- Fix createAgentsTab reference in App.tsx\n- Silence TypeScript lints by renaming unused vars (e.g., _error, _className)\n- Centralize project path selection in tab controls