refactor(dry-F1): shared useAsyncAction lifecycle helper for stores

New composables/useAsyncAction.js owns the loading/error/try-finally
lifecycle. Migrated 11 stores: credentials, downloads, sources, posts
(error=raw) + ml, artistDirectory, tagDirectory, showcase, suggestions,
seriesReader, modal (error=message). The errorAs option preserves each
store's existing error shape so store.error keeps the same type for
components (~50 consumption sites unchanged). Stores whose catch also
reset data (suggestions/seriesReader/modal) clear it upfront instead.

Deliberately NOT migrated (special control flow, would change behavior):
artist (conditional 404 catch + dual loading states), migration (rethrows),
gallery (inflight-id stale-response guard), and the Shape-B no-catch /
loaded-guard / keyed-cache stores.

Net -77 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/suggestions.js

@@ -2,6 +2,7 @@ import { defineStore } from 'pinia'
 import { toast } from '../utils/toast.js'
 import { ref } from 'vue'
 import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
 
 // Category display order: people/sources first, general last.
 export const CATEGORY_ORDER = ['artist', 'character', 'copyright', 'general']
@@ -15,23 +16,16 @@ export const CATEGORY_LABELS = {
 export const useSuggestionsStore = defineStore('suggestions', () => {
   const api = useApi()
   const byCategory = ref({})       // { category: [suggestion, ...] }
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
   let currentImageId = null
 
   async function load(imageId) {
     currentImageId = imageId
-    loading.value = true
-    error.value = null
-    try {
+    byCategory.value = {}  // cleared upfront so it stays empty on error
+    await run(async () => {
       const body = await api.get(`/api/images/${imageId}/suggestions`)
       byCategory.value = body.by_category || {}
-    } catch (e) {
-      error.value = e.message
-      byCategory.value = {}
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   function _drop(category, predicate) {