Rebuild as Lumen: private money by default on STRK20

Full pivot from the Aether analysis dashboard to a consumer money app:
- New product surface: home (dark-glass private balance), Pay / Receive /
  Add sheets, Spaces, People (relationship identities), private receipts,
  warned cash-out, sample-data preview at /app?preview
- The deanonymization engine now runs silently on the defender's side
  (guard.ts): amount tuning on deposits, exit-correlation and timing checks
  on cash-outs, cross-relationship reuse and cadence checks on payments
- New premium light design system, landing page, and README
- Rebrand everywhere: package, metadata, storage keys, Cairo contract
  (AetherSplitter -> LumenSplitter), GitHub repo and Vercel project renamed
  to lumen-strk20

Gates: tsc clean, eslint clean, 158 vitest + 29 snforge tests pass,
next build and scarb build succeed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>