Add end-user web-client design documentation

[TheSilkky]

Summary

• Add docs/end-user-web-client-design.md as design-only guidance for evolving the web client from technical prototype screens into end-user account, incident review, trusted-contact, viewer-link, and future fallback capture experiences.
• Link the design document from the README and docs index, and align architecture, theme, threat-model, viewer-token design, and changelog notes.
• Preserve current runtime behavior and keep future server-dependent behavior explicitly future-scoped.

Validation

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

Security And Scope Notes

• Docs-only change; no application code or backend behavior changes.
• Does not add browser decryption, recording or capture behavior, raw key handling, key escrow, notification delivery, emergency dispatch, or admin/operator UI.
• Keeps open-proofline/server as the backend source of truth.
• Issue number: none.