DRY pass #2, batch 5, F16: one way an action reports its failure (#5372)

composables/useAction.ts: toastOnFailure runs an action and toasts the
server's reason or a fallback; useAction adds the busy flag a button
waits on; useRowAction keeps the id of the row whose action is running.
Import, export, the menu entry, the integration prompt, sign out
elsewhere, the account reset link, the group actions (whose local act()
it replaces), invite revoke and sync disconnect each wrote that
try/catch/finally out.

Kept: AccountView's device revoke. It shows a fixed message rather than
the server's reason, and moving it would change the text. ShareDialog
shows its errors inline, not in a toast.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/GroupList.vue

@@ -2,10 +2,10 @@
 import { computed, onMounted, ref } from "vue";
 import { api } from "../api/client";
 import { errorMessage } from "../api/errors";
-import { useUiStore } from "../stores/ui";
 import BaseButton from "./BaseButton.vue";
 import BaseInput from "./BaseInput.vue";
 import Icon from "./Icon.vue";
+import { useRowAction } from "../composables/useAction";
 
 // Admin: groups of people a note can be shared with in one go (#5177). A group is
 // the instance's; anyone can share with it, and a note shared with it reaches whoever
@@ -23,7 +23,6 @@ interface Group {
   members: Person[];
 }
 
-const ui = useUiStore();
 
 const groups = ref<Group[]>([]);
 const accounts = ref<Person[]>([]);
@@ -31,7 +30,7 @@ const loading = ref(true);
 const error = ref("");
 const name = ref("");
 const creating = ref(false);
-const busy = ref("");
+const { busy, act } = useRowAction();
 // Which group's "Add someone" picker holds which account id.
 const picks = ref<Record<string, string>>({});
 
@@ -64,17 +63,6 @@ function replace(updated: Group) {
   groups.value = groups.value.map((g) => (g.id === updated.id ? updated : g)).sort(byName);
 }
 
-async function act(id: string, fn: () => Promise<void>, fallback: string) {
-  busy.value = id;
-  try {
-    await fn();
-  } catch (e) {
-    ui.showToast(errorMessage(e, fallback));
-  } finally {
-    busy.value = "";
-  }
-}
-
 async function create() {
   if (!name.value.trim()) return;
   creating.value = true;