feat(auth): staging wallet partagé — import assisté par fichier + e2e multi-navigateur

Stopgap staging multi-user sur wallet partagé (cf. brief_2026-06-15_shared-wallet-shim).

Distribution / import du wallet :
- AccessGateScreen : barrière d'accès ON PAR DÉFAUT (désactivable via
  globalThis.__FESTIPOD_ACCESS_GATE_DISABLED__ pour tests/dev). Fournit le FICHIER
  .ngw + le mot de passe + un guide en 3 étapes (import assisté sur nextgraph.eu —
  le broker hébergé n'autorise pas l'import inline pendant l'auth web-app).
- sharedWallet.ts + build.ts : fichier copié en /shared-wallet.ngw, mot de passe gravé.
- Ancien LoginScreen (/login) retiré ; atterrissage post-login -> /home.
- NextGraphContext : dé-piégeage de l'état "connecting" au retour (pageshow/bfcache).

Couche multistore stopgap : storeRegistry, isolation, AccountContext, FestipodDataContext.

Tests e2e multi-navigateur :
- browserPool + world.openBrowser : contextes frais isolés, 2 axes orthogonaux
  (nb de navigateurs × modèle de wallet own/shared).
- @humain : parcours humain complet (télécharge -> importe le fichier sur
  nextgraph.eu -> Entrer -> pseudo -> accueil).
- Bypass de la barrière pour @e2e via context.addInitScript.
- Convention @wip exclue via cucumber.json.

Docs (concepts) : nextgraph-platform (knowledge_broker-import-constraint,
decision_2026-06-17_assisted-wallet-import), bdd-testing (knowledge_multibrowser-harness).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/AuthGate.tsx

@@ -0,0 +1,48 @@
+/**
+ * AuthGate — the stopgap access flow (see decision_2026-06-15_shared-wallet-login-flow):
+ *   1. Technical access barrier (AccessGateScreen) → opens the SHARED wallet via
+ *      the broker redirect (with the wallet file + guide it hands the user).
+ *   2. Perceived app login (ConnexionScreen) → pick a username.
+ *   3. The app.
+ *
+ * The gate is ON BY DEFAULT (Festipod never functions without NextGraph). It is
+ * disabled only when `globalThis.__FESTIPOD_ACCESS_GATE_DISABLED__ === true` —
+ * injected by `build.ts` (from ACCESS_GATE_DISABLED=1) for a no-gate build, or
+ * by the test harness via `context.addInitScript` for @e2e (which exercises the
+ * screens, not the auth flow). Absent → gate ON.
+ */
+
+import type { ReactNode } from 'react';
+import { useNextGraph } from '../shared/context/NextGraphContext';
+import { useAccount } from '../shared/context/AccountContext';
+import { AccessGateScreen } from '../modules/auth/screens/AccessGateScreen';
+import { ConnexionScreen } from '../modules/auth/screens/ConnexionScreen';
+
+declare global {
+  // eslint-disable-next-line no-var
+  var __FESTIPOD_ACCESS_GATE_DISABLED__: boolean | undefined;
+}
+const GATE_DISABLED = globalThis.__FESTIPOD_ACCESS_GATE_DISABLED__ === true;
+
+export function AuthGate({ children }: { children: ReactNode }) {
+  const { status, error, connect } = useNextGraph();
+  const { username } = useAccount();
+
+  // Gate explicitly disabled (no-gate build / @e2e harness) → straight to app.
+  if (GATE_DISABLED) {
+    return <>{children}</>;
+  }
+
+  // 1. Technical access barrier (real NG login) — until the shared wallet opens.
+  if (status !== 'connected') {
+    return <AccessGateScreen status={status} error={error} onEnter={connect} />;
+  }
+
+  // 2. Perceived app login — until a username is chosen.
+  if (!username) {
+    return <ConnexionScreen />;
+  }
+
+  // 3. The app.
+  return <>{children}</>;
+}