feat(demo): DEMO_MODE lock, seed:demo and the demo banner (MODULE_API 1.18.0)

Core half of the public demo (docs/website/DEMO_MODE.md §10 step 1, website#222).
Nothing here runs unless DEMO_MODE=true.

- server/src/demo/: the lock at /api/v1, ahead of every router. It covers every
  Tier A row of §3.3, the shared-login conditions, POST /auth/register (DM16),
  DELETE /auth/me/sessions/:id on a demo account (DM17), and the PUT
  /admin/settings keys that reach a Tier A lock. Any multipart request is refused
  app-wide. noindex and robots.txt. A condition that cannot decide refuses.
- Loader: demoLocks in module.json (writes only, under the module's own mounts),
  ctx.demoMode, and api.onDemoSeed (DM15). MODULE_API 1.18.0 on both halves.
- GET /public/settings carries demo: { resetsAt, logins } on the demo only.
- npm run seed:demo -- <fixture>: upserts the four accounts, sets the site live
  with registration disabled, seeds content on the first run only, calls each
  module's onDemoSeed, and stamps demo_state. It resolves $env: strings and
  refuses to run off the demo.
- Client: the banner, one DEMO_LOCKED toast fed from the request primitive (so
  module screens get it too), and the published logins on both sign-in pages.
  Dashboard and register say the demo message instead of blaming a role.
- OpenAPI: DemoLocked schema, the demo block, and a note that any write may
  answer DEMO_LOCKED on the demo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

client/src/App.jsx

@@ -2,6 +2,8 @@ import { Routes, Route, Navigate, Outlet } from 'react-router-dom'
 import { AuthProvider } from './contexts/AuthContext.jsx'
 import { SiteProvider } from './contexts/SiteContext.jsx'
 import MaintenanceGate from './components/MaintenanceGate.jsx'
+import DemoBanner from './components/DemoBanner.jsx'
+import DemoToast from './components/DemoToast.jsx'
 import RequireAuth from './components/RequireAuth.jsx'
 import RequirePlayer from './components/RequirePlayer.jsx'
 import RoleGate from './components/RoleGate.jsx'
@@ -90,6 +92,10 @@ export default function App() {
             the provider hooks are called once for the whole app rather than
             once per screen. */}
         <ModuleFeaturesProvider>
+          {/* The public demo's strip and refusal toast (DEMO_MODE.md §3.1, §6).
+              Both render nothing unless the server says this is the demo. */}
+          <DemoBanner />
+          <DemoToast />
           <Routes>
             {/* Landing hero — always public, even in maintenance mode. The hero is
                 itself the pre-launch "coming soon" page, so it sits outside the