feat(web): derive page-header into canonical context-header with unified header contract [SPRINT-027]

Enhance ContextHeaderComponent to be the single canonical header primitive:
- Add configurable heading level (h1/h2/h3) for semantic HTML in nested shells
- Add testId input for Playwright targeting (data-testid)
- Add ARIA labels on return button and chip list (role=list/listitem)
- Add back-arrow indicator for improved return-button affordance
- Add JSDoc on all inputs for developer ergonomics

Deprecate PageHeaderComponent to a thin compatibility wrapper that delegates
to ContextHeaderComponent.

Adopt canonical header on 4 representative pages:
- RegistryAdminComponent (admin/setup surface)
- PackRegistryBrowserComponent (operational surface)
- DeadLetterDashboardComponent (operational surface)
- OfflineKitComponent (operational surface)

Each adopted page gains eyebrow breadcrumb context, consistent subtitle
placement, and projected actions via the shared header-actions slot,
replacing ~80 lines of repeated ad-hoc header markup.

15 focused component tests covering title rendering, eyebrow/subtitle
display, chips with ARIA, back action, action slot projection, heading
levels, testId, and responsive layout structure. All pass.

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

docs/modules/ui/TASKS.md

@@ -22,7 +22,7 @@
 - `docs/implplan/SPRINT_20260308_022_FE_unreachable_release_investigation_routes.md`
 - `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`
+- [DONE] `docs/implplan/SPRINT_20260308_027_FE_page_header_context_header_derivation.md` - Derived `PageHeaderComponent` into canonical `ContextHeaderComponent` with unified header contract, adopted on 4 target pages, 15 focused tests.
 - [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`
 - [DONE] `docs/implplan/SPRINT_20260308_030_FE_split_pane_list_detail_shell_consolidation.md` - Consolidated SplitPaneComponent into ListDetailShellComponent as the canonical master-detail layout primitive. Added collapsible toggle, detail slide-in animation, and accessibility roles. Adopted on signing-key-dashboard. SplitPaneComponent deprecated.