[P0] Consolidate the root Next app and `client/` Next app

[Calebux]

Summary

The repo contains both app/ and client/app/, which creates ambiguity around the canonical frontend.

Scope

• Area: architecture
• Priority: P0
• Backlog ID: #1

Acceptance Criteria

• One frontend entrypoint is declared canonical.
• Duplicate app tree is removed or archived.
• CI and docs reference only the canonical app.

Implementation Notes

• Keep the implementation aligned with current architecture decisions and security constraints in this repo.
• Include tests for changed behavior and update docs where relevant.

Definition of Done

• Acceptance criteria met
• Tests added/updated and passing
• Documentation updated
• No security regressions introduced

Links

• Backlog source: docs/repo-issue-backlog-2026-05.md