refactor(dashboard): harden default-tab derivation in useActiveDashboardTabs

Address review feedback on the embedded filter-bar fix:

- Replace bail-out returns that could leak undefined with EMPTY_ACTIVE_TABS
- Guard against undefined dashboardLayout in both useActiveDashboardTabs
  and useSelectChartTabParents (latent crash surfaced by new test)
- Drop redundant GRID_ID check; the TABS_TYPE check below already covers it
- Narrow queue.shift()! non-null assertion to an explicit guard
- Remove unused DASHBOARD_GRID_ID import
- Add 3 regression tests: GRID_ID-rooted layout, missing layout,
  deeply nested TABS

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>