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/decisions/2026-03-13-1400-conditional-ng-init-broker-detection.md

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+# Conditional NextGraph Init Based on Broker Iframe Detection
+
+**Date:** 2026-03-13 14:00
+**Status:** Accepted
+
+## Context
+
+`@ng-org/web`'s `initNgWeb()` checks `window.self === window.top`. When the app runs standalone (not in an iframe), it redirects the entire page to `nextgraph.net/redir/` to trigger broker authentication. This caused the app to redirect on every load — even during development or when the user hadn't clicked "Se connecter".
+
+## Options Considered
+
+### Option A: Always auto-init NG on mount
+**Arguments for:**
+- Simpler code — no branching logic
+
+**Arguments against:**
+- Causes immediate redirect to broker when loaded standalone
+- Breaks development workflow
+- User sees broker login page instead of the app
+
+### Option B: Conditional auto-init based on iframe detection
+**Arguments for:**
+- When in iframe, the broker has already authenticated — safe to auto-init
+- When standalone, user must explicitly click "Se connecter" to trigger the redirect
+- Preserves standalone demo/development experience
+- Matches `@ng-org/web`'s own detection logic
+
+**Arguments against:**
+- Relies on `window.self !== window.top` heuristic (could theoretically be wrong if embedded in non-broker iframe)
+
+## Decision
+
+Option B. `NextGraphContext` checks `const isInsideBroker = typeof window !== 'undefined' && window.self !== window.top` at module level. `useEffect` only auto-calls `initNg()` when `isInsideBroker` is true. The `connect()` callback remains available for explicit user-initiated connection.
+
+Additionally, `FestipodDataContext` now renders empty data (not seed data) during the `connecting` phase to avoid flashing demo content before the wallet loads.
+
+## Consequences
+
+**Positive:**
+- App loads without redirecting — works standalone for development and demo
+- In broker iframe, connection is seamless and automatic
+- No seed data flash during wallet connection
+
+**Negative:**
+- None significant
+
+**Risks:**
+- If `@ng-org/web` changes its detection logic, our guard may diverge — keep them aligned