frontend: upgrade ESLint 8 → 9 + typescript-eslint 7 → 8 (flat config)

Clears the six npm deprecation warnings that fire on every deploy:
inflight, @humanwhocodes/config-array, rimraf, glob,
@humanwhocodes/object-schema, and eslint 8.57 itself — all transitive
deps of the eslint v8 chain.

- eslint 8.57 → 9.10
- @typescript-eslint/{parser,eslint-plugin} 7.x → typescript-eslint 8.7
  (single combined package in v8)
- eslint-plugin-react-hooks 4 → 5 (v5 supports flat config)
- eslint-plugin-react 7.34 → 7.36
- Replace .eslintrc.cjs with eslint.config.js (flat config)
- `npm run lint` switches from --ext flag (legacy) to flat-config lookup

typecheck silent, lint silent, build identical bundle (45.93 KB raw,
15.76 KB gzip on the main entry).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/.eslintrc.cjs

@@ -1,24 +0,0 @@
-module.exports = {
-  root: true,
-  env: { browser: true, es2020: true },
-  extends: [
-    'eslint:recommended',
-    'plugin:@typescript-eslint/recommended',
-    'plugin:react/recommended',
-    'plugin:react-hooks/recommended',
-  ],
-  ignorePatterns: ['dist', '.eslintrc.cjs', 'vite.config.ts', 'postcss.config.js', 'tailwind.config.js'],
-  parser: '@typescript-eslint/parser',
-  parserOptions: {
-    ecmaVersion: 'latest',
-    sourceType: 'module',
-    ecmaFeatures: { jsx: true },
-  },
-  settings: { react: { version: '18.3' } },
-  plugins: ['react', 'react-hooks', '@typescript-eslint'],
-  rules: {
-    'react/react-in-jsx-scope': 'off',
-    'react/prop-types': 'off',
-    '@typescript-eslint/no-unused-vars': ['warn', { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }],
-  },
-};

frontend/eslint.config.js

@@ -0,0 +1,45 @@
+// Flat config — replaces the legacy .eslintrc.cjs. ESLint 9 + typescript-eslint 8.
+import js from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import react from 'eslint-plugin-react';
+import reactHooks from 'eslint-plugin-react-hooks';
+import globals from 'globals';
+
+export default tseslint.config(
+  {
+    ignores: [
+      'dist/**',
+      'node_modules/**',
+      'vite.config.ts',
+      'postcss.config.js',
+      'tailwind.config.js',
+      'eslint.config.js',
+    ],
+  },
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ['**/*.{ts,tsx}'],
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      parserOptions: { ecmaFeatures: { jsx: true } },
+      globals: { ...globals.browser, ...globals.es2020 },
+    },
+    plugins: {
+      react,
+      'react-hooks': reactHooks,
+    },
+    settings: { react: { version: '18.3' } },
+    rules: {
+      ...react.configs.recommended.rules,
+      ...reactHooks.configs.recommended.rules,
+      'react/react-in-jsx-scope': 'off',
+      'react/prop-types': 'off',
+      '@typescript-eslint/no-unused-vars': [
+        'warn',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+    },
+  },
+);

frontend/package.json

@@ -7,7 +7,7 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "preview": "vite preview",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint . --report-unused-disable-directives --max-warnings 0",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -20,18 +20,19 @@
     "recharts": "^2.12.0"
   },
   "devDependencies": {
+    "@eslint/js": "^9.10.0",
     "@types/react": "^18.3.0",
     "@types/react-dom": "^18.3.0",
-    "@typescript-eslint/eslint-plugin": "^7.16.0",
-    "@typescript-eslint/parser": "^7.16.0",
     "@vitejs/plugin-react": "^4.3.0",
     "autoprefixer": "^10.4.0",
-    "eslint": "^8.57.0",
-    "eslint-plugin-react": "^7.34.0",
-    "eslint-plugin-react-hooks": "^4.6.0",
+    "eslint": "^9.10.0",
+    "eslint-plugin-react": "^7.36.0",
+    "eslint-plugin-react-hooks": "^5.0.0",
+    "globals": "^15.9.0",
     "postcss": "^8.4.0",
     "tailwindcss": "^3.4.0",
     "typescript": "^5.5.0",
+    "typescript-eslint": "^8.7.0",
     "vite": "^5.3.0"
   }
 }