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/routes/player/PlayerRegister.jsx

@@ -3,6 +3,7 @@ import { Link, useNavigate } from 'react-router-dom'
 import ProviderIcon from '../../components/ProviderIcon.jsx'
 import { useAuth } from '../../contexts/AuthContext.jsx'
 import { api } from '../../api/client.js'
+import { isDemoLocked } from '../../lib/demoNotice.js'
 import PlayerShell, { honeypotStyle } from './PlayerShell.jsx'
 
 export default function PlayerRegister() {
@@ -54,6 +55,7 @@ export default function PlayerRegister() {
       navigate('/player', { replace: true })
     } catch (err) {
       if (err.status === 409) setError('That username is already taken.')
+      else if (isDemoLocked(err)) setError(err.message)
       else if (err.status === 403) setError('Registration is not open right now.')
       else if (err.status === 400) setError(err.message || 'Please check your details and try again.')
       else setError('Could not create your account right now.')