feat: open the door — no sign-in wall, ever

Point and shoot. SeancePage no longer redirects a stranger to /enter; it
silently provisions a wanderer (POST /auth/guest, a real account) and drops
them straight into the séance. The landing CTA goes to /seance
unconditionally. Registering is now what it should always have been: how
you *keep* a codex and unlock device pairing — not the toll to get in.

Details that matter:
- One auto-attempt only, guarded by a ref: survives StrictMode's double
  effect, and a failure (rate limit, offline) doesn't retry forever.
- The loading veil covers provisioning, so there's no flash of an empty
  séance while the account is created.
- If provisioning genuinely fails, it falls back to /enter rather than a
  blank screen — the manual door still exists.
- A returning visitor already holds a cookie, so this never fires for them.

App.test's "sends anonymous visitors to /enter" assertion was inverted to
assert the open door instead — that test encoded the wall we just removed.

Verified live: POST /auth/guest returns wanderer-a6f1 and that cookie
authenticates on /auth/me. 366 frontend tests pass; i18n parity holds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/pages/SeancePage.tsx

@@ -69,10 +69,28 @@ const TOAST_TTL_MS = 6000
 const SDR_SAMPLE_RATE_HZ = 2_048_000
 
 export function SeancePage() {
-  const { user, checking } = useAuth()
+  const { user, checking, guest } = useAuth()
   const { t } = useTranslation()
+  const [guestFailed, setGuestFailed] = useState(false)
+  // One auto-attempt only. Guards React StrictMode's double-effect and
+  // stops a failed attempt (rate limit / offline) from retrying forever.
+  const guestAttempted = useRef(false)
 
-  if (checking) {
+  // The open door. There is no sign-in wall: if nobody is authenticated
+  // once the session check settles, silently provision a wanderer — a real
+  // guest account (POST /auth/guest) — and drop straight into the séance.
+  // Point and shoot. Registering later is how a seeker *keeps* what they
+  // find and unlocks device pairing; it was never meant to be the toll to
+  // get in. A returning visitor already has a cookie, so `user` is set and
+  // this never fires for them.
+  useEffect(() => {
+    if (checking || user || guestAttempted.current) return
+    guestAttempted.current = true
+    void guest().catch(() => setGuestFailed(true))
+  }, [checking, user, guest])
+
+  // Still resolving a session, or provisioning the wanderer: hold the veil.
+  if (checking || (!user && !guestFailed)) {
     return (
       <div className="seance-loading" role="status">
         <div className="loading-sigil" aria-hidden>
@@ -82,6 +100,9 @@ export function SeancePage() {
       </div>
     )
   }
+
+  // Auto-provision failed (the veil is genuinely crowded, or offline). Fall
+  // back to the manual door rather than a blank screen or a crash.
   if (!user) return <Navigate to="/enter" replace />
 
   return (