🤝 Add contributor infrastructure: CONTRIBUTING.md, issue/PR templates, and GitHub Actions CI

## Summary
The repo has no contributor infrastructure: no `CONTRIBUTING.md`, no issue/PR templates, no CI, no linting. If we want outside contributors (which this issue backlog is for), the project needs guardrails so PRs arrive in reviewable shape and can't silently break the build.

## What to add

**1. `CONTRIBUTING.md`**
- How to run the app locally (`npm install && npm start`, Node 18+)
- How to compile/test contracts (`npx hardhat compile`, `npx hardhat test`)
- Branch/PR conventions and commit message expectations (recent history has commits like `cvb`, `jki`, `sty` — worth setting a bar)
- Where to ask questions

**2. `.github/` templates**
- `ISSUE_TEMPLATE/bug_report.yml` — repro steps, expected/actual, browser/device (this is a kids' PWA, mobile matters)
- `ISSUE_TEMPLATE/feature_request.yml`
- `PULL_REQUEST_TEMPLATE.md` — what/why, how tested, screenshots for UI changes

**3. GitHub Actions CI (`.github/workflows/ci.yml`)** — on every PR:
- `npm ci`
- `npx hardhat compile` (catches Solidity breakage)
- `npx hardhat test` (once the contract test suite exists — see the testing issue; wire it in even if the suite starts empty)
- ESLint on the JS files (add a minimal `eslint.config.js`; the codebase currently has no linting at all)
- Optional: a link-checker for markdown docs

**4. Labels** — seed the tracker with `good first issue`, `security`, `smart-contracts`, `frontend`, `docs` so the backlog is navigable.

## Notes for the implementer
- Keep CI fast (< 3 min) and free-tier friendly — single job, Node 18, npm cache
- ESLint will flag a lot in `game.js` on first run; start with `--max-warnings` high or lint only changed files, then ratchet down as the refactor issue lands
- `CONTRIBUTING.md` should link to the beginner-friendly issues

## Acceptance criteria
- Opening a PR triggers CI; a Solidity compile error or lint error fails the check
- New issues open with a structured template
- `CONTRIBUTING.md` gets a newcomer from clone to running game without reading any other doc

Start by reviewing the existing app commands and package configuration, then run `npm install && npm start`, `npx hardhat compile`, and `npx hardhat test`. Add `CONTRIBUTING.md`, the specified files under `.github/`, `.github/workflows/ci.yml`, and `eslint.config.js`; done means PRs trigger CI, issue forms are structured, and the contributor guide gets a newcomer from clone to running the game.

Written by the indexing model from the issue text.