Document viewer token UI design

[TheSilkky]

Summary

• Add a viewer-token UI design note for owner create/revoke flows and the planned web-client read-only viewer.
• Document the no-account notification/viewer-token boundary separately from future trusted-contact account/key flows.
• Update API, security, threat model, README, security policy, docs index, and changelog references.

Closes #52

Validation

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run test:e2e
• git diff --check