fix: make pinia API store references readonly

API store consumers shouldn’t be able to modify the internal state of
the store.

src/api.ts

 import { defineStore } from 'pinia'
-import { computed, ref, Ref } from 'vue'
+import { computed, readonly, ref, Ref } from 'vue'
 
 type ID = number | string
 
@@ -101,7 +101,10 @@ export function createExtendableAPI<T extends APIObject>(
   }
 
   return {
-    base: { items, error },
+    base: {
+      error: readonly(error),
+      items: readonly(items),
+    },
     createRequest,
     endpoint,
     maybeRaiseResponse,