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>

scripts/debug-browser.ts

@@ -0,0 +1,33 @@
+/**
+ * Manual debug tool: launches a headed Chromium to inspect NextGraph broker interactions.
+ * Creates a temporary profile in `.playwright-profile-debug/` (gitignored).
+ *
+ * Usage: bun scripts/debug-browser.ts
+ */
+import { chromium } from 'playwright';
+
+const browser = await chromium.launch({ headless: false });
+const context = await browser.newContext();
+const page = await context.newPage();
+
+console.log('Browser launched. Navigating to nextgraph.net...');
+
+page.on('pageerror', (err) => console.log('[pageerror]', err.message));
+page.on('close', () => console.log('[page closed]'));
+page.on('crash', () => console.log('[page crashed]'));
+browser.on('disconnected', () => console.log('[browser disconnected]'));
+
+try {
+  await page.goto('https://nextgraph.net/redir/#/?o=http%3A%2F%2F127.0.0.1%3A12345', {
+    waitUntil: 'domcontentloaded',
+    timeout: 30000,
+  });
+  console.log('Navigation done. Page URL:', page.url());
+} catch (err: any) {
+  console.log('Navigation error (expected):', err.message);
+}
+
+console.log('Waiting... close the browser manually when done.');
+
+// Keep alive indefinitely
+await new Promise(() => {});