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/implementation_plan.md

@@ -8,6 +8,7 @@ Provide a living plan for UI deliverables, dependencies, and evidence.
 - Update this file when new scoped work is approved.
 - Sprint `025` is active for safe cleanup of approved dead leaves and committed generated/debug artifacts in the Web workspace.
 - Sprint `026` shipped Settings IA rationalization: the Settings shell now owns only personal preferences (appearance, language, layout, AI assistant). All admin, tenant, and operations configuration leaves redirect to their canonical owners (Administration, Setup, Ops). See `docs/features/checked/web/settings-ia-rationalization-ui.md`.
+- Sprint `027` is DONE: derived `PageHeaderComponent` into canonical `ContextHeaderComponent` with unified header contract (configurable heading level, testId, ARIA), adopted on 4 target pages (RegistryAdmin, PackRegistryBrowser, DeadLetterDashboard, OfflineKit), 15 focused tests.
 - Sprint `031` (Witness Viewer Evidence Derivation) is DONE. Derived the orphan `WitnessViewerComponent` into 4 reusable proof-inspection sections (VerificationSummary, SignatureInspector, AttestationDetail, EvidencePayload) adopted on Reachability WitnessPage and Evidence PacketPage. See `docs/implplan/SPRINT_20260308_031_FE_witness_viewer_evidence_derivation.md`.
 
 ## Near-term deliverables