chore(eslint): migrate to ESLint 9 flat config format

- Add eslint.config.js with ESLint 9 flat config format
- Remove deprecated .eslintrc.cjs file
- Remove deprecated .eslintignore file (replaced by ignores in config)
- Maintain all existing ESLint rules and configurations
- Ensure compatibility with Husky 9.1.7

frontend/eslint.config.js

@@ -0,0 +1,78 @@
+import js from '@eslint/js'
+import tsParser from '@typescript-eslint/parser'
+import tsPlugin from '@typescript-eslint/eslint-plugin'
+import react from 'eslint-plugin-react'
+import unusedImports from 'eslint-plugin-unused-imports'
+import prettier from 'eslint-plugin-prettier'
+import globals from 'globals'
+
+export default [
+  {
+    ignores: [
+      'node_modules/',
+      'dist/',
+      'prettier.config.cjs',
+      '.eslintrc.cjs',
+      'env.d.ts',
+      'public/',
+      'assets/',
+      'vite-env.d.ts',
+      '.prettierignore',
+      'package-lock.json',
+      'package.json',
+      'postcss.config.cjs',
+      'tailwind.config.cjs',
+      'tsconfig.json',
+      'tsconfig.node.json',
+      'vite.config.ts',
+    ],
+  },
+  {
+    files: ['**/*.{js,jsx,ts,tsx}'],
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      parser: tsParser,
+      parserOptions: {
+        ecmaFeatures: {
+          jsx: true,
+        },
+      },
+      globals: {
+        ...globals.browser,
+        ...globals.es2021,
+        ...globals.node,
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tsPlugin,
+      react,
+      'unused-imports': unusedImports,
+      prettier,
+    },
+    rules: {
+      ...js.configs.recommended.rules,
+      ...tsPlugin.configs.recommended.rules,
+      ...react.configs.recommended.rules,
+      ...prettier.configs.recommended.rules,
+      'react/prop-types': 'off',
+      'unused-imports/no-unused-imports': 'error',
+      'react/react-in-jsx-scope': 'off',
+      'no-undef': 'off',
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-unused-vars': 'warn',
+      '@typescript-eslint/no-unused-expressions': 'warn',
+      'prettier/prettier': [
+        'error',
+        {
+          endOfLine: 'auto',
+        },
+      ],
+    },
+    settings: {
+      react: {
+        version: 'detect',
+      },
+    },
+  },
+]