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/showcase.js

@@ -1,32 +1,26 @@
 import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
 
 const PAGE = 60
 
 export const useShowcaseStore = defineStore('showcase', () => {
   const api = useApi()
   const images = ref([])
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
   const exhausted = ref(false)
   const seen = new Set()
 
   async function fetchPage() {
     if (loading.value) return
-    loading.value = true
-    error.value = null
-    try {
+    await run(async () => {
       const body = await api.get('/api/showcase', { params: { limit: PAGE } })
       const fresh = body.images.filter(i => !seen.has(i.id))
       if (fresh.length === 0) { exhausted.value = true; return }
       for (const i of fresh) seen.add(i.id)
       images.value.push(...fresh)
-    } catch (e) {
-      error.value = e.message
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   async function shuffle() {