Add ESLint for TypeScript/React linting

- Install eslint, typescript-eslint, eslint-plugin-react-hooks
- Configure eslint.config.js with flat config format
- Add type: module to package.json
- Fix unused variable issues (prefix with underscore)
- Add Makefile targets: lint-frontend, fix-frontend

frontend/eslint.config.js

@@ -0,0 +1,31 @@
+import eslint from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import reactHooks from 'eslint-plugin-react-hooks';
+
+export default tseslint.config(
+  eslint.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    plugins: {
+      'react-hooks': reactHooks,
+    },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+      // Downgrade to warnings - existing patterns use these
+      'react-hooks/exhaustive-deps': 'warn',
+      'react-hooks/set-state-in-effect': 'off',
+    },
+  },
+  {
+    ignores: [
+      '.next/**',
+      'node_modules/**',
+      'app/generated/**',
+    ],
+  }
+);
+