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>

.project/knowledge/bdd-testing.md

@@ -15,9 +15,9 @@ Each module has step directories for three test layers:
 
 ```
 src/modules/event/steps/
-  ui/           # UI/screen assertions (active)
-  data/         # Data layer assertions
-  e2e/          # Full integration (planned)
+  ui/           # UI/screen assertions (source analysis)
+  data/         # Data layer assertions (Playwright + broker)
+  e2e/          # E2E assertions (Playwright + broker + real app UI)
 ```
 
 Shared steps (cross-domain) live in `src/shared/steps/ui/`.
@@ -97,6 +97,15 @@ Run all: `bun run test:cucumber`
 
 `@data` scenarios test through the real NextGraph broker. See [data-layer-testing](./data-layer-testing.md) for full architecture.
 
+## E2E Testing
+
+`@e2e` scenarios test the real app running in the broker iframe. See [data-layer-testing](./data-layer-testing.md#e2e-layer) for architecture. Key differences from `@data`:
+
+- Uses the **real app** (not a test harness) served on a local HTTP port
+- Interacts via Playwright locators and `evaluate()` on the app iframe
+- Tests actual UI behavior: navigation, redirects, button clicks, screen content
+- Requires real broker mode (fails with `Error` if broker unavailable)
+
 ## Adding New Steps
 
 1. **Module-specific**: Create in `src/modules/{module}/steps/ui/`