feat(web): derive metric-card into canonical KPI card with semantic delta handling [SPRINT-028]

Rework MetricCardComponent from a basic label+value+delta card into the
canonical Stella Ops KPI card primitive with:

- deltaDirection input ('up-is-good' | 'up-is-bad' | 'neutral') to control
  green/red semantics per metric context
- severity input ('healthy' | 'warning' | 'critical' | 'unknown') for
  left-border health accents
- unit input for display units (ms, %, /hr, GB)
- loading, empty, and error states with skeleton/placeholder rendering
- ARIA accessibility (role="group", composite aria-label, delta labels)
- Responsive dense-grid support

Adopted on 3 representative dashboards (12 bespoke tiles replaced):
- signals-runtime-dashboard (3 cards)
- search-quality-dashboard (4 cards)
- delivery-analytics (5 cards)

40 focused tests covering delta direction semantics, all states, severity
accents, and accessibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/modules/ui/TASKS.md

@@ -21,7 +21,7 @@
 - `docs/implplan/SPRINT_20260308_023_FE_unreachable_registry_admin_route.md`
 - `docs/implplan/SPRINT_20260308_026_FE_settings_information_architecture_rationalization.md`
 - `docs/implplan/SPRINT_20260308_027_FE_page_header_context_header_derivation.md`
-- `docs/implplan/SPRINT_20260308_028_FE_metric_card_dashboard_card_derivation.md`
+- [DONE] `docs/implplan/SPRINT_20260308_028_FE_metric_card_dashboard_card_derivation.md` - Derived MetricCardComponent into canonical KPI card with semantic delta handling, severity accents, and loading/empty/error states. Adopted on 3 dashboards (12 bespoke tiles replaced). 40 tests pass.
 - `docs/implplan/SPRINT_20260308_029_FE_timeline_list_audit_timeline_derivation.md`
 - `docs/implplan/SPRINT_20260308_030_FE_split_pane_list_detail_shell_consolidation.md`
 - `docs/implplan/SPRINT_20260308_031_FE_witness_viewer_evidence_derivation.md`