fix: Add ESLint configuration and fix code style issues

Created ESLint configuration and auto-fixed code style violations to
resolve CI/CD pipeline linting failures.

## Problem

CI/CD pipeline was failing at the linting step:
```
ESLint couldn't find a configuration file.
```

The project had ESLint installed but no configuration, causing the lint
step in the pipeline to fail.

## Solution

### 1. Created .eslintrc.json

**Configuration Details:**
- Environment: Browser, ES2021, Node
- Extends: eslint:recommended
- Parser: ES Modules, latest ECMAScript
- Rules:
  - 4-space indentation
  - Unix line endings
  - Single quotes (with escape allowance)
  - Semicolons required
  - Unused vars as warnings (prefixed with _ ignored)
  - Console allowed (common in development)

### 2. Auto-Fixed Code Issues

Ran `eslint --fix` to automatically resolve:
- ✅ 16 indentation errors (standardized to 4 spaces)
- ✅ Formatting inconsistencies
- ✅ Code style violations

**Remaining:**
- 6 warnings for unused parameters (acceptable, won't fail CI)
- These are interface parameters maintained for consistency

## Files Modified

- .eslintrc.json (new) - ESLint configuration
- js/engine/MoveValidator.js - indentation fixes
- js/engine/SpecialMoves.js - indentation fixes
- js/main.js - style fixes
- js/pieces/King.js - formatting
- js/pieces/Piece.js - formatting

## Verification

```bash
npm run lint
✖ 6 problems (0 errors, 6 warnings)
```

✅ No errors - pipeline will pass
⚠️ 6 warnings - informational only, don't fail build

## Impact

✅ CI/CD linting step will now succeed
✅ Consistent code style across project
✅ Automated style checking on all commits
✅ Better code readability and maintainability

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.eslintrc.json

@@ -0,0 +1,28 @@
+{
+  "env": {
+    "browser": true,
+    "es2021": true,
+    "node": true
+  },
+  "extends": "eslint:recommended",
+  "parserOptions": {
+    "ecmaVersion": "latest",
+    "sourceType": "module"
+  },
+  "rules": {
+    "indent": ["error", 4],
+    "linebreak-style": ["error", "unix"],
+    "quotes": ["error", "single", { "avoidEscape": true }],
+    "semi": ["error", "always"],
+    "no-unused-vars": ["warn", { "argsIgnorePattern": "^_" }],
+    "no-console": "off",
+    "no-debugger": "warn"
+  },
+  "ignorePatterns": [
+    "node_modules/",
+    "coverage/",
+    "dist/",
+    "build/",
+    "*.min.js"
+  ]
+}