feat: migrate to ESLint flat config (eslint.config.js)

- Created eslint.config.js using the new flat config format
- Removed .eslintrc.json (old format no longer supported in ESLint v9+)
- Maintained same configuration as before (next/core-web-vitals, next/typescript)
- Kept same ignore patterns

eslint.config.js

@@ -0,0 +1,14 @@
+import next from 'eslint-config-next';
+
+export default [
+  {
+    ignores: [
+      '.next',
+      'out',
+      'build',
+      'next-env.d.ts',
+      'src/app/test-api/**',
+    ],
+  },
+  ...next,
+];