feat: add code coverage setup with Codecov integration and GitHub badge

[Copilot]

No code coverage reporting infrastructure existed despite having a unit:test:coverage script — no thresholds, no CI upload, no badge.

Description

Sets up Jest coverage collection, enforces thresholds, uploads reports to Codecov on every CI run, and surfaces coverage via a README badge.

What change does this PR introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes to docs/code comments)

What is the proposed approach?

jest.config.js — Coverage configuration added:

• collectCoverageFrom: scopes to src/**/*.{ts,tsx}, excluding dev/entry-point files (App.tsx, devIndex.tsx, index.tsx, viteconfig.ts, *.d.ts)
• coverageReporters: ['text', 'lcov', 'json-summary']: console output + lcov for Codecov + json-summary for tooling
• coverageThreshold: CI fails if coverage drops below these measured-and-conservative baselines:

coverageThreshold: {
  global: {
    statements: 75,  // current ~79.3%
    branches: 55,    // current ~61.1%
    functions: 65,   // current ~74.1%
    lines: 75,       // current ~79.1%
  },
}

codecov.yml — Codecov project config:

• Project target ≥75% (2% drop tolerance), patch target ≥70% (5% tolerance)
• PR comments showing diff/flags/files breakdown
• unit flag scoped to src/

.github/workflows/test.yml — CI changes:

• npm run unit:test → npm run unit:test:coverage to generate the lcov report
• Uploads ./coverage/lcov.info via codecov/codecov-action@v5 with the unit flag
• CODECOV_TOKEN secret forwarded through workflow_call

.github/workflows/lint-and-build.yml — Passes CODECOV_TOKEN secret to the reusable test workflow.

README.md — Codecov badge added next to the CI badge.

Setup required: Add CODECOV_TOKEN as a GitHub Actions secret after connecting the repo at codecov.io (free for open source).

On the "monorepo / multi-stack" question: this repo has two test stacks (Jest unit + Cypress e2e). The industry-standard approach is to upload each as a separate flagged report to Codecov, which merges them into a unified view. This PR covers the Jest side. Cypress coverage (via @cypress/code-coverage) can be added as a follow-up with an integration flag.

Checklist:

• The commit message follows our adopted guidelines
• Testing has been done for the change(s) added (for bug fixes/features)
• Relevant comments/docs have been added/updated (for bug fixes/features)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• download.cypress.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node dist/index.js --exec install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)