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>

.gitignore

@@ -33,3 +33,8 @@ report.[0-9]_.[0-9]_.[0-9]_.[0-9]_.json
 # Finder (MacOS) folder config
 .DS_Store
 .ng-tarballs
+
+# Playwright persistent profile (contains NG wallet)
+.playwright-profile/
+.playwright-profile-debug/
+playwright/.auth/