## What
Adds a `DEMO_ENABLED` env var (`config('app.demo.enabled')`, default
`true`) to fully toggle the demo account. Setting `DEMO_ENABLED=false`
in production blocks it without code changes.
When disabled:
- **Login is blocked** — `Fortify::authenticateUsing` rejects the demo
account with a generic credentials error (doesn't reveal the demo is
off). Regular users and 2FA are unaffected.
- **Landing link hidden** — the "Check Demo" button on the landing page
is removed (shared `demoEnabled` prop).
- **No credential prefill** — `demoCredentials` is only shared when
enabled, so `/login?demo=1` no longer autofills.
## Why
The demo account is publicly shared and gets abused (e.g. duplicate
votes on integration requests). This gives us a kill switch.
## Tests
Added to `DemoAccountRestrictionsTest`:
- demo account cannot log in when disabled
- demo account can log in when enabled
- regular user can still log in when demo is disabled
Existing auth + 2FA tests still pass.
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