Every preview shares the origin minds.gamecult.org and so one localStorage. A login another
preview left (cinny_hs_base_url of a real homeserver) made the demo connect to that server, which
the guard refuses, so SpecVersions showed 'Unable to connect to the homeserver'. installDemo now
drops any stored session whose base is not the demo origin, and the service worker is registered
after that so it is never handed the dropped session. Replaces the foreign-session banner text.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>