chore(lint): baseline root ESLint config + root lint script wired into CI (#16)

[Vyacheslav-Tomashevskiy]

Closes #16.

Problem

The root eslint.config.js exported a flat config with only an ignores entry — no rules, plugins, or language options — so eslint . at the repo root checked nothing. The root lint script (pnpm --recursive --stream lint) never linted root-level JS either, and CI only lints inside frontend/. Root config/tooling files were completely unlinted.

Changes in this PR

• eslint.config.js — added a dependency-free baseline ruleset (no-unused-vars, no-undef, no-var, prefer-const, eqeqeq) scoped to root-level JS via a files glob, with node globals declared. backend/, frontend/ and contract/ are ignored so the root pass never double-lints sub-packages that own their own ESLint configs and lint steps (per the Out of scope note).
• package.json — root lint script is now eslint ., so it lints root-level JS and exits non-zero on violations.

No new dependencies are added: the config uses only the already-installed eslint, so the committed pnpm-lock.yaml is unchanged and --frozen-lockfile installs keep working.

CI wiring (.github/workflows/frontend-ci-cd.yml)

To make CI actually invoke root linting (Files to touch item), a lint-root job should run pnpm run lint at the repo root and be added to the ci-status aggregate. I could not push this file myself — my git token intentionally lacks the workflow scope, so GitHub rejects PAT pushes that touch .github/workflows/. I've enabled maintainer can modify; the exact change is below and I'm happy to have it applied to the branch:

# add after the lint-and-test job:
  lint-root:
    name: Root ESLint
    runs-on: ubuntu-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v4
      - name: Setup pnpm
        uses: pnpm/action-setup@v4
      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version: "20"
          cache: "pnpm"
          cache-dependency-path: "./pnpm-lock.yaml"
      - name: Install root dependencies
        run: pnpm install --frozen-lockfile
      - name: Run root ESLint
        run: pnpm run lint

# and add lint-root to the ci-status aggregate:
#   needs: [lint-and-test, lint-root, build]
#   for job in lint-and-test lint-root build; do

Acceptance criteria

• Root ESLint config applies a baseline rule set to root-level JS files
• A root lint script exists and exits non-zero on violations (eslint .)
• Approach consistent with how CI invokes linting (pnpm run lint; lint-root job above)
• No package's existing lint behavior is broken (sub-packages ignored; their own lint steps untouched)

Verification (eslint 9)

• eslint.config.js lints clean (exit 0).
• A root JS file with var / unused var / == fails the run (exit 1).
• Files under the ignored sub-package dirs are skipped.
• The only root-level JS file in the repo is eslint.config.js itself, so the lint-root job is green.

/claim #16

RTC payout address: RTCd1554f0f35576faf01d386a6be1c947f560dd0b7