chore(deps): upgrade ESLint to v9 with flat config

- Upgrade eslint from v8 to v9.39.2
- Upgrade typescript-eslint to v8.52.0 (unified package)
- Add angular-eslint package for flat config support
- Migrate from .eslintrc.json to eslint.config.js (flat config)
- Update package configs for sync-md, automations, boilerplate-solid-js
- Remove unused eslint-disable directives
- Fix lint errors (empty interface, template eqeqeq)

BREAKING: Requires Node.js 18.18+ (ESLint 9 requirement)

eslint.config.js

@@ -0,0 +1,127 @@
+// @ts-check
+const tseslint = require('typescript-eslint');
+const angular = require('angular-eslint');
+const prettierRecommended = require('eslint-plugin-prettier/recommended');
+const preferArrow = require('eslint-plugin-prefer-arrow');
+
+module.exports = tseslint.config(
+  // Global ignores
+  {
+    ignores: [
+      'app-builds/**/*',
+      'dist/**',
+      'node_modules/**/*',
+      'src/app/t.const.ts',
+      'src/assets/bundled-plugins/**/*',
+      'src/app/config/env.generated.ts',
+      '.tmp/**/*',
+      'packages/**/*',
+    ],
+  },
+  // TypeScript files
+  {
+    files: ['**/*.ts'],
+    extends: [
+      ...tseslint.configs.recommended,
+      ...angular.configs.tsRecommended,
+      prettierRecommended,
+    ],
+    processor: angular.processInlineTemplates,
+    plugins: {
+      'prefer-arrow': preferArrow,
+    },
+    languageOptions: {
+      parserOptions: {
+        ecmaVersion: 2020,
+        sourceType: 'module',
+      },
+    },
+    rules: {
+      // Disabled rules
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/ban-ts-comment': 'off',
+      '@typescript-eslint/no-empty-function': 'off',
+      '@typescript-eslint/explicit-module-boundary-types': 'off',
+      '@angular-eslint/component-selector': 'off',
+      '@angular-eslint/no-input-rename': 'off',
+      '@typescript-eslint/no-inferrable-types': 'off',
+      'no-underscore-dangle': 'off',
+      'arrow-body-style': 'off',
+      '@typescript-eslint/member-ordering': 'off',
+      'import/order': 'off',
+      'arrow-parens': 'off',
+      '@typescript-eslint/explicit-member-accessibility': 'off',
+
+      // Enabled rules
+      'prettier/prettier': 'error',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { args: 'none', caughtErrors: 'none' },
+      ],
+      '@typescript-eslint/explicit-function-return-type': [
+        'error',
+        {
+          allowExpressions: true,
+          allowTypedFunctionExpressions: true,
+          allowHigherOrderFunctions: true,
+          allowDirectConstAssertionInArrowFunctions: true,
+          allowConciseArrowFunctionExpressionsStartingWithVoid: true,
+        },
+      ],
+      '@typescript-eslint/naming-convention': [
+        'error',
+        {
+          selector: 'default',
+          format: ['camelCase', 'snake_case', 'UPPER_CASE', 'PascalCase'],
+          leadingUnderscore: 'allowSingleOrDouble',
+          trailingUnderscore: 'allow',
+          filter: { regex: '(should)|@tags', match: false },
+        },
+        {
+          selector: 'variable',
+          format: ['camelCase', 'snake_case', 'UPPER_CASE', 'PascalCase'],
+          leadingUnderscore: 'allowSingleOrDouble',
+          trailingUnderscore: 'allow',
+        },
+        { selector: 'enum', format: ['PascalCase', 'UPPER_CASE'] },
+        { selector: 'typeLike', format: ['PascalCase'] },
+      ],
+      'prefer-const': 'error',
+      '@typescript-eslint/no-unused-expressions': 'error',
+      '@typescript-eslint/no-empty-object-type': 'error',
+      'max-len': [
+        'error',
+        {
+          ignorePattern: '^import \\{.+;$',
+          ignoreRegExpLiterals: true,
+          ignoreStrings: true,
+          ignoreUrls: true,
+          code: 150,
+        },
+      ],
+      'id-blacklist': 'error',
+      // @typescript-eslint/member-delimiter-style removed in v8 - Prettier handles this
+      'no-shadow': 'off',
+      '@typescript-eslint/no-shadow': 'error',
+      'comma-dangle': ['error', 'always-multiline'],
+      'no-mixed-operators': 'error',
+      'prefer-arrow/prefer-arrow-functions': 'error',
+      '@angular-eslint/directive-selector': [
+        'error',
+        { type: 'attribute', prefix: '', style: 'camelCase' },
+      ],
+      // @typescript-eslint/ban-types replaced by specific rules in v8
+      '@typescript-eslint/no-unsafe-function-type': 'error',
+      '@typescript-eslint/no-wrapper-object-types': 'error',
+    },
+  },
+  // HTML files
+  {
+    files: ['**/*.html'],
+    extends: [...angular.configs.templateRecommended, prettierRecommended],
+    rules: {
+      '@angular-eslint/template/no-negated-async': 'off',
+      'prettier/prettier': 'error',
+    },
+  },
+);