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/controllers/details_controller.js

@@ -1,4 +1,5 @@
 import { Controller } from "@hotwired/stimulus"
+import { disableElement, enableElement } from "element_helpers"
 
 export default class extends Controller {
   static targets = ["countLabel", "submitButton", "list"]
@@ -18,10 +19,10 @@ export default class extends Controller {
     const count = this.element.querySelectorAll('input:checked:not([disabled])').length
     if (count > 0) {
       this.countLabelTarget.textContent = count + ' selected'
-      Turbo.StreamElement.prototype.enableElement(this.submitButtonTarget)
+      enableElement(this.submitButtonTarget)
     } else {
       this.countLabelTarget.textContent = this.countLabelTarget.dataset.prompt
-      Turbo.StreamElement.prototype.disableElement(this.submitButtonTarget)
+      disableElement(this.submitButtonTarget)
     }
   }