E2E testing infrastructure, NextGraph connection fixes, and documentation

- Add @e2e test layer: real app in broker iframe via Playwright
- Fix broker redirect: conditional auto-init only when inside iframe
- Fix seed data flash: empty data during 'connecting' phase
- Fix Gallery button in iframe: explicit navigate instead of history.back
- Add auth e2e feature scenarios and step definitions
- Update docs: bdd-testing, data-layer-testing, data-layer, AGENTS.md
- Add decision record for conditional NG init approach

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

src/app/App.tsx

@@ -14,7 +14,7 @@ function AppContent() {
     return (
       <DemoMode
         initialScreenId={route.screenId}
-        onBack={goBack}
+        onBack={() => navigate({ page: 'gallery' })}
         onNavigateToStory={(storyId) => navigate({ page: 'specs', storyId })}
       />
     );