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>

package.json

@@ -4,11 +4,13 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "dev": "portless run bun --hot src/index.ts",
+    "dev": "portless festipod bun --hot src/index.ts",
     "start": "NODE_ENV=production bun src/index.ts",
     "build": "bun run build.ts",
     "test:cucumber": "bun run cucumber:run && bun run cucumber:report && bun run features:parse && bun run steps:extract",
     "cucumber:run": "node --import tsx/esm node_modules/.bin/cucumber-js --config cucumber.json",
+    "test:data": "node --import tsx/esm node_modules/.bin/cucumber-js --config cucumber.json --tags @data",
+    "test:auth-setup": "bun scripts/setup-test-auth.ts",
     "cucumber:report": "bun scripts/parse-test-results.ts",
     "features:parse": "bun scripts/parse-features.ts",
     "steps:extract": "bun scripts/extract-step-definitions.ts",
@@ -42,6 +44,7 @@
     "@types/react-dom": "^19",
     "chai": "^6.2.2",
     "happy-dom": "^16.6.0",
+    "playwright": "^1.58.2",
     "tailwindcss": "^4.1.11",
     "tsx": "^4.21.0",
     "tw-animate-css": "^1.4.0"