Migrate ESLint config to the new v9 format

See:
 - https://eslint.org/docs/latest/use/migrate-to-9.0.0#flat-config
 - https://eslint.org/docs/latest/use/configure/migration-guide
 - https://eslint.org/docs/latest/use/configure/configuration-files#configuration-file

Did:
 > npx @eslint/migrate-config .eslintrc.json
 > npm install @eslint/js @eslint/eslintrc -D
 > mv eslint.config.mjs eslint.config.js

# Conflicts:
#	package-lock.json

eslint.config.js

@@ -0,0 +1,21 @@
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all,
+});
+
+export default [
+  ...compat.extends(
+    "prettier",
+    "eslint:recommended",
+    "plugin:@typescript-eslint/recommended",
+    "plugin:@next/next/core-web-vitals",
+  ),
+];

package.json

@@ -22,6 +22,8 @@
     "recharts": "^2.13.3"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.1.0",
+    "@eslint/js": "^9.14.0",
     "@types/d3": "^7.4.3",
     "@types/jsdom": "^21.1.7",
     "@types/node": "^22",