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/views/SyncView.vue

@@ -24,6 +24,7 @@ import {
 } from "../desktop/bridge";
 import { errorMessage } from "../api/errors";
 import { formatDateTime } from "../notes/datetime";
+import { toastOnFailure } from "../composables/useAction";
 
 // Opt-in server sync for the desktop app. Being UNLINKED is the normal resting
 // state, not an incomplete setup — the app is local-first and fully usable having
@@ -319,7 +320,7 @@ async function disconnect() {
   ) {
     return;
   }
-  try {
+  await toastOnFailure(async () => {
     const result = await syncBridge.unlink();
     status.value = result.status;
     linkedAs.value = "";
@@ -331,9 +332,7 @@ async function disconnect() {
         ? "Disconnected, and this device is signed out on the server."
         : "Disconnected. This device now works offline only.",
     );
-  } catch (e) {
-    ui.showToast(errorMessage(e, "Couldn't disconnect."));
-  }
+  }, "Couldn't disconnect.");
 }
 
 // A background cycle finished — redraw from it, so this screen is never staler