feat(#2443): asyncBtn — shared reusable component, replace ad-hoc disable/spinner patterns

add `asyncBtn(btn, fn)` to `@hive/shared/forms.js` as the single
reusable component for async button actions:
  1. double-click guard: returns immediately if btn is already disabled
  2. saves btn.innerHTML, replaces with spinner while in-flight
  3. restores btn on resolve or reject via finally

wire it into all ad-hoc disable/spinner/restore patterns:
  - common.js: bindAsyncForms uses asyncBtn internally
  - core.js: 'clear perms' button
  - permissions.js: clearStaleAgent
  - schedules.js: saveSchedule submit, editSchedule submit
  - app.js: buildAnswerForm, buildInboxMarkAllRow

fireScheduleNow in schedules.js is left with its existing childNode
save/restore because it shows a custom result flash on the button
content after a successful fire-now (the auto-restore of asyncBtn
would overwrite it); the surrounding themedConfirm dialog already
acts as a natural double-click barrier before the fetch.

saveAll in permissions.js is also left as-is: it uses a custom
'queued ✓' success label + a 900ms delay before re-fetch; the
btn.dataset.busy flag is its own double-submit guard.

frontend/packages/shared/src/forms.js

@@ -0,0 +1,34 @@
+// Shared async-button primitive. Used by both the dashboard and the
+// per-agent UI for any button that triggers a network action.
+//
+// `asyncBtn(btn, fn)` — the single reusable component:
+//   1. Guards double-click: returns immediately if `btn` is already
+//      disabled (prevents a second identical request from firing).
+//   2. Saves `btn.innerHTML` and replaces it with a spinner during the
+//      async operation.
+//   3. Re-enables the button and restores the original content when `fn`
+//      resolves or rejects (via `finally`), so callers don't need
+//      save/restore boilerplate.
+//
+// Usage:
+//   btn.addEventListener('click', () => asyncBtn(btn, async () => {
+//     const resp = await fetch('/api/...');
+//     if (!resp.ok) throw new Error(await resp.text());
+//     // handle success
+//   }));
+//
+// Error handling: `asyncBtn` restores the button on any thrown error /
+// rejected promise but does NOT surface the error — callers must catch
+// and display it themselves (via `themedToast`, `alert`, a status span,
+// etc.) before the re-throw, or handle it inside `fn` without
+// re-throwing.
+export function asyncBtn(btn, fn) {
+  if (btn.disabled) return; // double-click guard
+  const orig = btn.innerHTML;
+  btn.disabled = true;
+  btn.innerHTML = '<span class="spinner">◐</span>';
+  fn().finally(() => {
+    btn.disabled = false;
+    btn.innerHTML = orig;
+  });
+}