fix(client): Handle empty groups and API paths

[IsaacCheng9]

Summary

• Added an empty-group client path
  • Kept the initial screen usable when /users returned no records
  • Made the first created user become the active user
• Centralised client API routing
  • Replaced hardcoded client-side localhost calls with relative API paths
  • Kept REACT_APP_API_URL available for non-proxy deployments
• Expanded frontend regression coverage
  • Covered empty database loading and first-user creation with mocked API calls
  • Fixed the case-sensitive component import that would fail on Linux
• Added client verification to CI
  • Ran the client test suite and production build alongside the existing backend job
  • Synchronised client/package-lock.json so npm 10 can use npm ci
• Stabilised the backend duplicate-user test
  • Waited for Mongoose user indexes before asserting duplicate-key rejection

Rationale

• The app previously assumed a populated user collection
  • A fresh database followed the README setup path into users[0].username, crashing before anyone could create the first user
• Relative API paths match the existing CRA proxy setup
  • This removed the localhost:3000 / localhost:3001 split without committing the project to one deployment host
• Mongoose unique constraints are index-backed
  • The duplicate-user regression test could run before the in-memory Mongo indexes were ready, making CI nondeterministic