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/package.json

@@ -14,7 +14,8 @@
     "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",
     "./terminal.css": "./src/terminal.css",
-    "./chrome.css": "./src/chrome.css"
+    "./chrome.css": "./src/chrome.css",
+    "./forms.js": "./src/forms.js"
   },
   "files": [
     "src/"