fix: guard AddToCollectionModal against out-of-order load results

Retargeting the modal mid-load could resolve responses out of order, overwriting
the newer target's membership with the older one's (audit FE-2). Route load()
through the tested CancellableLoader so a superseded result is dropped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.128.14"
+version = "0.128.15"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.128.14"
+version = "0.128.15"
 edition = "2021"
 default-run = "mangalord"
 

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.14",
+  "version": "0.128.15",
   "private": true,
   "type": "module",
   "scripts": {

frontend/src/lib/components/AddToCollectionModal.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
     import AdaptiveDialog from './AdaptiveDialog.svelte';
+    import { CancellableLoader } from '$lib/util/cancellable';
     import {
         addMangaToCollection,
         createCollection,
@@ -74,19 +75,30 @@
             : removePageFromCollection(collectionId, t.id);
     }
 
+    // Reopening for a different target (or an in-flight retarget) fires load()
+    // again; without this guard the two responses can resolve out of order and
+    // the first target's membership overwrites the second's (audit FE-2). The
+    // CancellableLoader drops a superseded call's result — the newer load then
+    // owns both the state and the spinner.
+    const loader = new CancellableLoader();
+
     async function load() {
         loading = true;
         error = null;
         try {
+            const result = await loader.run(async () => {
                 const [page, ids] = await Promise.all([
                     listMyCollections({ limit: 200 }),
                     loadContaining(target)
                 ]);
-            collections = page.items;
-            containingIds = new Set(ids);
+                return { items: page.items, ids };
+            });
+            if (result === null) return; // superseded — a newer load owns the state
+            collections = result.items;
+            containingIds = new Set(result.ids);
+            loading = false;
         } catch (e) {
             error = (e as Error).message;
-        } finally {
             loading = false;
         }
     }