add linter for checking license headers (and eslint --fix target to match, and fix missing license headers while we're here) (#62)

eslint.config.js

@@ -12,6 +12,7 @@ import reactHooks from 'eslint-plugin-react-hooks';
 import prettierConfig from 'eslint-config-prettier';
 import importPlugin from 'eslint-plugin-import';
 import globals from 'globals';
+import licenseHeader from 'eslint-plugin-license-header';
 
 export default tseslint.config(
   {
@@ -142,6 +143,24 @@ export default tseslint.config(
       'default-case': 'error',
     },
   },
+  {
+    files: ['./**/*.{tsx,ts,js}'],
+    plugins: {
+      'license-header': licenseHeader,
+    },
+    rules: {
+      'license-header/header': [
+        'error',
+        [
+          '/**',
+          ' * @license',
+          ' * Copyright 2025 Google LLC',
+          ' * SPDX-License-Identifier: Apache-2.0',
+          ' */',
+        ],
+      ],
+    },
+  },
   // Prettier config must be last
   prettierConfig,
 );