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/seriesReader.js

@@ -1,6 +1,7 @@
 import { defineStore } from 'pinia'
 import { ref } from 'vue'
 import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
 
 // IR reader.js "viewport-center-in-third" rule, pure for unit tests.
 // metrics: [{page_number, top, height}] in page order.
@@ -33,23 +34,16 @@ export const useSeriesReaderStore = defineStore('seriesReader', () => {
 
   const series = ref(null)
   const pages = ref([])
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
 
   async function load(tagId) {
-    loading.value = true
-    error.value = null
-    try {
+    series.value = null  // cleared upfront so they stay reset on error
+    pages.value = []
+    await run(async () => {
       const body = await api.get(`/api/series/${tagId}/pages`)
       series.value = body.series
       pages.value = body.pages
-    } catch (e) {
-      error.value = e.message
-      series.value = null
-      pages.value = []
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   return { series, pages, loading, error, load }