Data-layer BDD testing infrastructure and steps/frontend → steps/ui rename

- Rename steps/frontend/ to steps/ui/ across all modules and shared
- Add data-layer test harness (mock + real broker modes) with Playwright
- Add inscription data-layer steps (@data scenarios)
- Add test auth setup script and browser debug script
- Update docs (architecture, BDD testing, data-layer testing)
- Add ADR for headless wallet creation decision

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

AGENTS.md

@@ -28,6 +28,8 @@ NextGraph (P2P/local-first) with SHEX shapes and ORM. See [data-layer](.project/
 
 Multi-layer Cucumber/Gherkin in French. See [bdd-testing](.project/knowledge/bdd-testing.md).
 
+`@data` scenarios test through the real NextGraph broker with Playwright. See [data-layer-testing](.project/knowledge/data-layer-testing.md).
+
 ## Quick Start
 
 ```bash
@@ -45,3 +47,4 @@ bun run build:orm        # Regenerate ORM from SHEX shapes
 - [Data Layer](.project/knowledge/data-layer.md) — NextGraph, shapes, context, seed data
 - [BDD Testing](.project/knowledge/bdd-testing.md) — Cucumber setup, step layers, feature files
 - [Screens](.project/knowledge/screens.md) — screen inventory, registry, sketchy components
+- [Data-Layer Testing](.project/knowledge/data-layer-testing.md) — real broker testing, wallet setup, Playwright harness