feat(frontend): OIDC login + runtime env (FABRIC_OIDC_ONLY/FIX_TO_CENTER)

- Runtime container env injected by docker/entrypoint.sh -> runtime-env.js
  (loaded before the bundle); src/lib/runtime-env.ts reads it.
  FABRIC_OIDC_ONLY hides the password form; FIX_TO_CENTER pins the
  Center base and hides its input. Dockerfile ENTRYPOINT + ENV defaults.
- LoginPage: 'Sign in with SSO' when /auth/oidc/status enabled; password
  form gated by OIDC_ONLY; center input gated by FIX_TO_CENTER.
- /oidc route (OidcCallback) redeems the fragment ticket via
  /auth/oidc/exchange and adopts the session (AuthContext.adoptSession).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/OidcCallback.tsx

@@ -0,0 +1,66 @@
+import { useEffect, useRef, useState } from 'react'
+import { useNavigate } from 'react-router-dom'
+import { useAuth } from '../auth/auth-context'
+import { APP_NAME } from '../lib/brand'
+import { resolveCenterBase } from '../lib/runtime-env'
+import { oidcExchangeCenter } from '../lib/center-auth-client'
+
+// Landing route for the OIDC redirect: Center bounces the browser here
+// with #oidc_ticket=... (or #oidc_error=...). We redeem the one-time
+// ticket for a full session and adopt it.
+export default function OidcCallback() {
+  const navigate = useNavigate()
+  const { adoptSession } = useAuth()
+  const [error, setError] = useState('')
+  const ran = useRef(false)
+
+  useEffect(() => {
+    if (ran.current) return
+    ran.current = true
+
+    const hash = window.location.hash.replace(/^#/, '')
+    const params = new URLSearchParams(hash)
+    const ticket = params.get('oidc_ticket')
+    const errParam = params.get('oidc_error')
+    // scrub the fragment so the ticket isn't left in the URL/history
+    window.history.replaceState(null, '', window.location.pathname)
+
+    if (errParam) {
+      setError(decodeURIComponent(errParam))
+      return
+    }
+    if (!ticket) {
+      setError('Missing OIDC ticket.')
+      return
+    }
+
+    oidcExchangeCenter(resolveCenterBase(), ticket)
+      .then((next) => {
+        adoptSession(next)
+        navigate('/workspace', { replace: true })
+      })
+      .catch(() => setError('SSO sign-in failed. The ticket may have expired — please try again.'))
+  }, [adoptSession, navigate])
+
+  return (
+    <div className="login-screen">
+      <div className="login-card">
+        <div className="brand-wordmark">{APP_NAME}</div>
+        {error ? (
+          <>
+            <h1>Sign-in failed</h1>
+            <p className="error-text" style={{ marginBottom: 16 }}>{error}</p>
+            <button className="btn" type="button" onClick={() => navigate('/login', { replace: true })}>
+              Back to sign in
+            </button>
+          </>
+        ) : (
+          <>
+            <h1>Signing you in…</h1>
+            <p className="login-sub">Completing SSO authentication.</p>
+          </>
+        )}
+      </div>
+    </div>
+  )
+}