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>

js/engine/MoveValidator.js

@@ -226,8 +226,8 @@ export class MoveValidator {
 
         // King can't pass through check
         for (let col = king.position.col + direction;
-             col !== targetCol + direction;
-             col += direction) {
+            col !== targetCol + direction;
+            col += direction) {
 
             const simulatedBoard = board.clone();
             simulatedBoard.movePiece(king.position.row, king.position.col, row, col);

js/engine/SpecialMoves.js

@@ -155,20 +155,20 @@ export class SpecialMoves {
 
         let newPiece;
         switch (pieceType) {
-            case 'queen':
-                newPiece = new Queen(color, { row, col });
-                break;
-            case 'rook':
-                newPiece = new Rook(color, { row, col });
-                break;
-            case 'bishop':
-                newPiece = new Bishop(color, { row, col });
-                break;
-            case 'knight':
-                newPiece = new Knight(color, { row, col });
-                break;
-            default:
-                newPiece = new Queen(color, { row, col });
+        case 'queen':
+            newPiece = new Queen(color, { row, col });
+            break;
+        case 'rook':
+            newPiece = new Rook(color, { row, col });
+            break;
+        case 'bishop':
+            newPiece = new Bishop(color, { row, col });
+            break;
+        case 'knight':
+            newPiece = new Knight(color, { row, col });
+            break;
+        default:
+            newPiece = new Queen(color, { row, col });
         }
 
         board.setPiece(row, col, newPiece);