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/implplan/SPRINT_20260308_027_FE_page_header_context_header_derivation.md

@@ -20,7 +20,7 @@
 ## Delivery Tracker
 
 ### FE-PHD-001 - Freeze the canonical header contract
-Status: TODO
+Status: DONE
 Dependency: none
 Owners: UX, Developer (FE)
 Task description:
@@ -28,12 +28,12 @@ Task description:
 - Document which capabilities remain mandatory: contextual eyebrow, chips, back action, action slot strategy, supportive note, and responsive stacking behavior.
 
 Completion criteria:
-- [ ] A single canonical header API is defined.
-- [ ] Unused or redundant `PageHeaderComponent` behavior is either absorbed or rejected explicitly.
-- [ ] Header semantics are described in UX terms, not only implementation terms.
+- [x] A single canonical header API is defined.
+- [x] Unused or redundant `PageHeaderComponent` behavior is either absorbed or rejected explicitly.
+- [x] Header semantics are described in UX terms, not only implementation terms.
 
 ### FE-PHD-002 - Derive the reusable header primitive
-Status: TODO
+Status: DONE
 Dependency: FE-PHD-001
 Owners: Developer (FE)
 Task description:
@@ -41,12 +41,12 @@ Task description:
 - Keep the API small and expressive; avoid two near-identical shared header components.
 
 Completion criteria:
-- [ ] The canonical header primitive supports the required title, metadata, and action variants.
-- [ ] `PageHeaderComponent` is either removed or reduced to a compatibility wrapper with a clear migration path.
-- [ ] Header behavior remains responsive and accessible.
+- [x] The canonical header primitive supports the required title, metadata, and action variants.
+- [x] `PageHeaderComponent` is either removed or reduced to a compatibility wrapper with a clear migration path.
+- [x] Header behavior remains responsive and accessible.
 
 ### FE-PHD-003 - Adopt the derived header on target pages
-Status: TODO
+Status: DONE
 Dependency: FE-PHD-002
 Owners: Developer (FE), UX
 Task description:
@@ -54,33 +54,40 @@ Task description:
 - Use adoption to prove the pattern works for both dense operator surfaces and simpler settings/admin pages.
 
 Completion criteria:
-- [ ] At least one simple settings/admin page and one richer operational page adopt the derived header pattern.
-- [ ] Repeated header markup is removed from adopted surfaces.
-- [ ] The adopted pages gain clearer context and action placement.
+- [x] At least one simple settings/admin page and one richer operational page adopt the derived header pattern.
+- [x] Repeated header markup is removed from adopted surfaces.
+- [x] The adopted pages gain clearer context and action placement.
 
 ### FE-PHD-004 - Verify, document, and retire the orphan path
-Status: TODO
+Status: DONE
 Dependency: FE-PHD-003
 Owners: Test Automation, Documentation author
 Task description:
 - Add focused tests for the canonical header behavior and record the derivation decision in UI docs so future reviews treat the old generic header as intentionally superseded.
 
 Completion criteria:
-- [ ] Component or host tests cover the canonical header behavior.
-- [ ] UI docs explain the header derivation and adoption targets.
-- [ ] The old orphan path is no longer ambiguous in the shared inventory.
+- [x] Component or host tests cover the canonical header behavior.
+- [x] UI docs explain the header derivation and adoption targets.
+- [x] The old orphan path is no longer ambiguous in the shared inventory.
 
 ## Execution Log
 | Date (UTC) | Update | Owner |
 | --- | --- | --- |
 | 2026-03-08 | Sprint created to derive the unused generic page header into the mounted context-header pattern and adopt one canonical header primitive. | Codex |
+| 2026-03-08 | FE-PHD-001: Frozen the canonical header contract. `ContextHeaderComponent` is the single canonical header. `PageHeaderComponent` had only title, subtitle, and action slots; all useful bits absorbed. Canonical API: title (required), eyebrow (optional), subtitle (optional), contextNote (optional), chips (optional status indicators), backLabel+backClick (optional return action), headingLevel (1/2/3 for semantic HTML), testId (optional), header-actions content projection slot. | Developer (FE) |
+| 2026-03-08 | FE-PHD-002: Enhanced `ContextHeaderComponent` with configurable heading level (h1/h2/h3), testId, arrow in return button, ARIA labels on return button and chip list, JSDoc on all inputs. `PageHeaderComponent` reduced to deprecated compatibility wrapper delegating to `ContextHeaderComponent`. | Developer (FE) |
+| 2026-03-08 | FE-PHD-003: Adopted canonical header on 4 target pages: `RegistryAdminComponent` (admin/setup page), `PackRegistryBrowserComponent` (operational page), `DeadLetterDashboardComponent` (operational page), `OfflineKitComponent` (operational page). Removed repeated ad-hoc header markup from all 4. Each page now has eyebrow breadcrumb, consistent subtitle, and projected actions via the shared header. | Developer (FE) |
+| 2026-03-08 | FE-PHD-004: Added 15 focused component tests covering title rendering, eyebrow/subtitle display, chips with ARIA roles, back action behavior, action slot projection, heading level configurability (h1/h2/h3), testId attribute, and responsive layout structure. All 15 pass. Updated sprint and docs. Marked `PageHeaderComponent` as deprecated in the shared index. | Test Automation |
 
 ## Decisions & Risks
-- Decision target: one canonical header primitive, not parallel “simple” and “contextual” header abstractions.
+- **Decision: Single canonical header.** `ContextHeaderComponent` is the sole canonical header primitive. `PageHeaderComponent` is deprecated to a thin compatibility wrapper.
+- **Decision: Heading level configurability.** Added `headingLevel` input (1, 2, or 3) to support pages nested inside shells that already provide an h1. Default remains h1.
+- **Decision: Back button arrow.** Added a left arrow indicator to the return button for improved affordance and accessibility.
+- **Decision: testId support.** Added `testId` input that maps to `data-testid` on the header element for Playwright/test targeting.
+- **Decision: Adopted pages.** Registry Admin (admin/setup), Pack Registry Browser (operational), Dead-Letter Dashboard (operational), Offline Kit (operational). These four prove the pattern works across both simple admin and richer operational surfaces.
 - Risk: overfitting the header API to too many page variants could make the primitive hard to use.
-- Mitigation: validate the API on a small adoption set before broad rollout.
+- Mitigation: validated the API on a bounded 4-page adoption set. Future rollout should proceed incrementally.
 
 ## Next Checkpoints
-- Freeze the canonical header contract.
-- Prototype the derived shared header.
-- Adopt it on a bounded set of mounted pages.
+- Broader rollout of canonical header to remaining pages with ad-hoc headers (not scoped to this sprint).
+- Eventual removal of `PageHeaderComponent` once no references remain.