Landing: split demo vs mainnet WITHOUT a build env var

[danielgorgonha]

Single landing, two buttons (See demo = mock, Open vault = real), routed by path/runtime in the same deploy. Refactor the demo gate from build-time VITE_DEMO to runtime.

[danielgorgonha]

Code half done in #83 (merged): demo/real decided at RUNTIME (?demo=1 / ?demo=0, persisted; VITE_DEMO fallback), NET=helperConfigured() && !DEMO so one canonical build serves both. Landing CTA enters demo mode; DemoBanner has an Exit link. Live-validated. Remaining (Vercel dashboard, not code): consolidate to one canonical deploy, retire/redirect the stale net-preview, point the submission demo URL at ?demo=1 or keep VITE_DEMO=1.