Extract disableElement/enableElement to shared module

Stimulus controllers were reaching into Turbo.StreamElement.prototype
to call disableElement/enableElement — tight coupling to Turbo internals.

Extract both functions to app/javascript/element_helpers.js and import
from there in application.js (which still assigns them to the Turbo
prototype for server-driven Turbo Stream actions), details_controller,
and readout_unit_controller.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/javascript/application.js

@@ -2,6 +2,7 @@
 // https://github.com/rails/importmap-rails
 import "@hotwired/turbo-rails"
 import "controllers"
+import { disableElement, enableElement } from "element_helpers"
 
 
 /* Hide page before loaded for testing purposes */
@@ -12,23 +13,13 @@ document.addEventListener('turbo:load', showPage)
 
 
 /* Turbo stream actions */
-Turbo.StreamElement.prototype.disableElement = function(element) {
-    element.setAttribute("disabled", "disabled")
-    element.setAttribute("aria-disabled", "true")
-    element.setAttribute("tabindex", "-1")
-}
+Turbo.StreamElement.prototype.disableElement = disableElement
+Turbo.StreamElement.prototype.enableElement = enableElement
 Turbo.StreamActions.disable = function() {
-  this.targetElements.forEach((e) => { this.disableElement(e) })
-}
-
-Turbo.StreamElement.prototype.enableElement = function(element) {
-    element.removeAttribute("disabled")
-    element.removeAttribute("aria-disabled")
-    // Assume 'tabindex' is not used explicitly, so removing it is safe
-    element.removeAttribute("tabindex")
+  this.targetElements.forEach(disableElement)
 }
 Turbo.StreamActions.enable = function() {
-  this.targetElements.forEach((e) => { this.enableElement(e) })
+  this.targetElements.forEach(enableElement)
 }
 
 /* TODO: change to visibility = collapse to avoid width change? */