feat(web): consolidate split-pane into list-detail-shell as canonical master-detail layout [SPRINT-030]

Extend ListDetailShellComponent with collapsible toggle button, detail panel
slide-in animation, and accessibility roles (complementary, aria-controls,
focus-visible). Adopt on signing-key-dashboard (trust-admin) for side-by-side
key list + detail browsing. Deprecate SplitPaneComponent. Add 15 focused
component tests covering rendering, toggle behavior, and accessibility.

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

docs/modules/ui/implementation_plan.md

@@ -14,6 +14,7 @@ Provide a living plan for UI deliverables, dependencies, and evidence.
 - The queued orphan batch currently spans `SPRINT_20260308_013` through `SPRINT_20260308_023` and is intentionally not marked active until product review approves staffing.
 - Newly queued follow-on planning sprints cover Settings information architecture rationalization plus UX derivation tracks for the orphan `PageHeaderComponent`, `MetricCardComponent`, `TimelineListComponent`, `SplitPaneComponent`, and `WitnessViewerComponent` (`SPRINT_20260308_026` through `SPRINT_20260308_031`).
 - Sprint `028` (MetricCardComponent derivation into canonical KPI card) is DONE. The shared `MetricCardComponent` now supports semantic delta direction (`up-is-good` / `up-is-bad` / `neutral`), severity accents, loading/empty/error states, and ARIA accessibility. Adopted on signals-runtime, search-quality, and delivery-analytics dashboards. See `docs/implplan/SPRINT_20260308_028_FE_metric_card_dashboard_card_derivation.md`.
+- Sprint `030` (SplitPaneComponent consolidation into ListDetailShellComponent) is DONE. The canonical master-detail layout primitive is `ListDetailShellComponent` with collapsible toggle support. `SplitPaneComponent` is deprecated. Adopted on signing-key-dashboard (trust-admin). See sprint file for contract details.
 - Sprint `014` (CopyToClipboard, InlineCode, TruncatePipe adoption) is DONE. See `docs/features/checked/web/orphan-copy-inline-truncate-adoption.md`.
 - Sprint `015` (FilterBarComponent adoption) shipped, then was partially rolled back on audit-family pages to restore lost filter semantics. See `docs/features/checked/web/filter-bar-unification.md` and `docs/features/checked/web/orphan-revival-regression-remediation-ui.md`.
 - Sprint `020` (FindingListComponent consolidation) shipped, then was rolled back on mounted findings and release-security hosts because the shared contract required fabricated data. See `docs/features/checked/web/orphan-finding-list-consolidation.md` and `docs/features/checked/web/orphan-revival-regression-remediation-ui.md`.