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/sources.js

@@ -1,42 +1,30 @@
 import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
 
 export const useSourcesStore = defineStore('sources', () => {
   const api = useApi()
 
   // keyed cache: null => all sources, number => artist_id filtered
   const byArtist = ref(new Map())
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction()
   const scheduleStatus = ref(null)
 
   const allSources = computed(() => byArtist.value.get(null) ?? [])
 
   async function loadAll() {
-    loading.value = true
-    error.value = null
-    try {
+    await run(async () => {
       const body = await api.get('/api/sources')
       byArtist.value.set(null, body)
-    } catch (e) {
-      error.value = e
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   async function loadForArtist(artistId) {
-    loading.value = true
-    error.value = null
-    try {
+    await run(async () => {
       const body = await api.get('/api/sources', { params: { artist_id: artistId } })
       byArtist.value.set(artistId, body)
-    } catch (e) {
-      error.value = e
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   function _invalidate(artistId) {