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/downloads.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'
 
 export const useDownloadsStore = defineStore('downloads', () => {
   const api = useApi()
@@ -13,8 +14,7 @@ export const useDownloadsStore = defineStore('downloads', () => {
     from_date: null, to_date: null,
   })
   const selected = ref(null)
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction()
   const stats = ref({ pending: 0, running: 0, ok: 0, error: 0, skipped: 0 })
   const activity = ref({ hours: 24, buckets: [] })
   const failing = ref([])
@@ -28,33 +28,22 @@ export const useDownloadsStore = defineStore('downloads', () => {
   }
 
   async function loadFirst() {
-    loading.value = true
-    error.value = null
-    try {
+    await run(async () => {
       const body = await api.get('/api/downloads', { params: _params() })
       events.value = body
       cursor.value = body.length ? body[body.length - 1].id : null
       hasMore.value = body.length === 50
-    } catch (e) {
-      error.value = e
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   async function loadMore() {
     if (!hasMore.value || cursor.value == null) return
-    loading.value = true
-    try {
+    await run(async () => {
       const body = await api.get('/api/downloads', { params: _params({ before: cursor.value }) })
       events.value.push(...body)
       cursor.value = body.length ? body[body.length - 1].id : cursor.value
       hasMore.value = body.length === 50
-    } catch (e) {
-      error.value = e
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   async function loadOne(id) {