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/modal.js

@@ -2,14 +2,14 @@ import { defineStore } from 'pinia'
 import { toast } from '../utils/toast.js'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
 
 export const useModalStore = defineStore('modal', () => {
   const api = useApi()
 
   const currentImageId = ref(null)
   const current = ref(null)
-  const loading = ref(false)
-  const error = ref(null)
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
 
   // Post-scoped cycle. When set, prev/next cycles within this array
   // (used by PostCard's expanded-mosaic PostImageGrid clicks). When
@@ -20,8 +20,7 @@ export const useModalStore = defineStore('modal', () => {
 
   async function open (id, opts = {}) {
     currentImageId.value = id
-    loading.value = true
-    error.value = null
+    current.value = null  // cleared upfront so it stays null on error
     // Update post-scoped state if caller passed it; otherwise clear so
     // the next open() from gallery context uses neighbors mode.
     if (opts.postImageIds != null) {
@@ -32,14 +31,9 @@ export const useModalStore = defineStore('modal', () => {
       postImageIds.value = null
       postImageIndex.value = 0
     }
-    try {
+    await run(async () => {
       current.value = await api.get(`/api/gallery/image/${id}`)
-    } catch (e) {
-      error.value = e.message
-      current.value = null
-    } finally {
-      loading.value = false
-    }
+    })
   }
 
   async function close () {