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/composables/useAsyncAction.js

@@ -0,0 +1,28 @@
+import { ref } from 'vue'
+
+// Owns the loading/error lifecycle shared by most Pinia stores. `run`
+// flips loading on, clears error, runs `fn`, captures any throw into error
+// (swallowed so callers can read `store.error`), and always resets loading.
+// Returns fn's resolved value on success, or undefined if it threw.
+//
+// `errorAs` matches each store's existing convention so migrations don't
+// change what `store.error` holds: 'raw' keeps the thrown object (the
+// ApiError, with .status/.body), 'message' keeps just the string.
+export function useAsyncAction({ errorAs = 'raw' } = {}) {
+  const loading = ref(false)
+  const error = ref(null)
+
+  async function run(fn) {
+    loading.value = true
+    error.value = null
+    try {
+      return await fn()
+    } catch (e) {
+      error.value = errorAs === 'message' ? (e?.message ?? String(e)) : e
+    } finally {
+      loading.value = false
+    }
+  }
+
+  return { loading, error, run }
+}