fix: fix lint by using polyfill

frontend/eslint-polyfill.mjs

@@ -0,0 +1,13 @@
+// Polyfill for Object.groupBy (requires Node.js 21+, we're on 20)
+// This must be imported before any code that uses Object.groupBy
+if (typeof Object.groupBy === 'undefined') {
+  Object.groupBy = (items, keyFn) => {
+    const result = {}
+    let index = 0
+    for (const item of items) {
+      const key = keyFn(item, index++)
+      ;(result[key] ??= []).push(item)
+    }
+    return result
+  }
+}

frontend/eslint.config.mjs

@@ -1,4 +1,5 @@
 // @ts-check
+import './eslint-polyfill.mjs'
 import withNuxt from './.nuxt/eslint.config.mjs'
 
 import js from '@eslint/js'