Migrate ESLint from v8 to v9 with flat config

Upgraded ESLint to v9.39.4 and migrated configuration from the legacy
.eslintrc.json format to the new flat config format (eslint.config.mjs).

Changes:
- Updated eslint from ^8.57.1 to ^9.17.0
- Created eslint.config.mjs with flat config format
- Removed .eslintrc.json (legacy format)
- Removed .eslintignore (now using ignores in config)
- Migrated all existing rules and settings to flat config
- Used .mjs extension to avoid module type warnings

Config migration details:
- Converted extends/plugins to ES module imports
- Moved ignorePatterns to top-level ignores property
- Converted env/globals to languageOptions.globals
- Maintained all existing rules and settings
- Added prettier config as final config object

Result:
- All linting passing
- All tests passing
- Production build working
- No ESLint warnings

Author: dduugg

.eslintignore

@@ -0,0 +1,82 @@
+import tseslint from '@typescript-eslint/eslint-plugin';
+import tsparser from '@typescript-eslint/parser';
+import importPlugin from 'eslint-plugin-import';
+import eslintConfigPrettier from 'eslint-config-prettier';
+
+export default [
+  {
+    ignores: [
+      'out/**',
+      'dist/**',
+      '**/*.d.ts',
+      'coverage/**',
+      'node_modules/**',
+    ],
+  },
+  {
+    files: ['**/*.ts', '**/*.tsx', '**/*.js', '**/*.jsx'],
+    languageOptions: {
+      parser: tsparser,
+      parserOptions: {
+        project: './tsconfig.eslint.json',
+        sourceType: 'module',
+      },
+      ecmaVersion: 2020,
+      sourceType: 'module',
+      globals: {
+        console: 'readonly',
+        process: 'readonly',
+        __dirname: 'readonly',
+        __filename: 'readonly',
+        module: 'readonly',
+        require: 'readonly',
+        exports: 'readonly',
+        Buffer: 'readonly',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tseslint,
+      import: importPlugin,
+    },
+    rules: {
+      '@typescript-eslint/adjacent-overload-signatures': 'error',
+      '@typescript-eslint/no-empty-function': 'error',
+      '@typescript-eslint/no-empty-interface': 'warn',
+      '@typescript-eslint/no-namespace': 'error',
+      '@typescript-eslint/prefer-for-of': 'warn',
+      '@typescript-eslint/triple-slash-reference': 'error',
+      '@typescript-eslint/unified-signatures': 'warn',
+      'no-param-reassign': 'error',
+      'import/no-unassigned-import': 'warn',
+      'comma-dangle': ['error', 'only-multiline'],
+      'constructor-super': 'error',
+      eqeqeq: ['warn', 'always'],
+      'no-cond-assign': 'error',
+      'no-duplicate-case': 'error',
+      'no-duplicate-imports': 'error',
+      'no-empty': [
+        'error',
+        {
+          allowEmptyCatch: true,
+        },
+      ],
+      'spaced-comment': 'error',
+      'no-invalid-this': 'error',
+      'no-new-wrappers': 'error',
+      'no-redeclare': 'error',
+      'no-sequences': 'error',
+      'no-shadow': [
+        'error',
+        {
+          hoist: 'all',
+        },
+      ],
+      'no-throw-literal': 'error',
+      'no-unsafe-finally': 'error',
+      'no-unused-labels': 'error',
+      'no-var': 'warn',
+      'prefer-const': 'warn',
+    },
+  },
+  eslintConfigPrettier,
+];