Remove duplicate disable/enable logic and fetch() calls

readoutUnitChanged was manually setting disabled/aria-disabled/tabindex
attributes — duplicating Turbo.StreamElement.prototype.disableElement/
enableElement which already exists for this purpose. Replace with calls
to those methods.

Also replace fetch() in setDefaultUnit and drop with form.requestSubmit()
so Turbo handles CSRF, stream responses and lifecycle natively.

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

app/javascript/application.js

@@ -82,40 +82,34 @@ window.detailsObserver = new MutationObserver((mutations) => {
 function readoutUnitChanged(select) {
   var button = select.closest('tr').querySelector('.set-default-unit');
   if (select.value && select.value !== select.dataset.defaultUnitId) {
-    button.removeAttribute('disabled');
-    button.removeAttribute('aria-disabled');
-    button.removeAttribute('tabindex');
+    Turbo.StreamElement.prototype.enableElement(button);
   } else {
-    button.setAttribute('disabled', 'disabled');
-    button.setAttribute('aria-disabled', 'true');
-    button.setAttribute('tabindex', '-1');
+    Turbo.StreamElement.prototype.disableElement(button);
   }
 }
 window.readoutUnitChanged = readoutUnitChanged
 
 function setDefaultUnit(button) {
   var select = button.closest('tr').querySelector('select[data-default-unit-id]');
-  var params = new URLSearchParams();
-  params.append('quantity[default_unit_id]', select.value);
-
-  fetch(button.dataset.path, {
-    body: params,
-    headers: {
-      'Accept': 'text/vnd.turbo-stream.html',
-      'X-CSRF-Token': document.head.querySelector('meta[name=csrf-token]').content,
-      'X-Requested-With': 'XMLHttpRequest'
-    },
-    method: 'PATCH'
-  })
-  .then(response => {
-    if (response.ok) {
+  var form = document.createElement('form');
+  form.action = button.dataset.path;
+  form.method = 'post';
+  form.dataset.turboStream = 'true';
+  var methodInput = document.createElement('input');
+  methodInput.type = 'hidden'; methodInput.name = '_method'; methodInput.value = 'patch';
+  var unitInput = document.createElement('input');
+  unitInput.type = 'hidden'; unitInput.name = 'quantity[default_unit_id]'; unitInput.value = select.value;
+  form.appendChild(methodInput);
+  form.appendChild(unitInput);
+  form.addEventListener('turbo:submit-end', function(event) {
+    if (event.detail.success) {
       select.dataset.defaultUnitId = select.value;
       readoutUnitChanged(select);
     }
-    return response.text();
-  })
-  .then(html => Turbo.renderStreamMessage(html))
-  .catch(err => console.error('setDefaultUnit failed:', err));
+    form.remove();
+  });
+  document.body.appendChild(form);
+  form.requestSubmit();
 }
 window.setDefaultUnit = setDefaultUnit
 
@@ -288,22 +282,17 @@ window.dragEnd = dragEnd
 
 function drop(event) {
   event.preventDefault()
-
-  var params = new URLSearchParams()
-  var id_param = event.currentTarget.getAttribute("data-drop-id-param")
+  var idParam = event.currentTarget.getAttribute("data-drop-id-param")
   var id = event.currentTarget.getAttribute("data-drop-id").split("_").pop()
-  params.append(id_param, id)
-
-  fetch(event.dataTransfer.getData("text/plain"), {
-    body: params,
-    headers: {
-      "Accept": "text/vnd.turbo-stream.html",
-      "X-CSRF-Token": document.head.querySelector("meta[name=csrf-token]").content,
-      "X-Requested-With": "XMLHttpRequest"
-    },
-    method: "POST"
-  })
-  .then(response => response.text())
-  .then(html => Turbo.renderStreamMessage(html))
+  var form = document.createElement('form');
+  form.action = event.dataTransfer.getData("text/plain");
+  form.method = 'post';
+  form.dataset.turboStream = 'true';
+  var input = document.createElement('input');
+  input.type = 'hidden'; input.name = idParam; input.value = id;
+  form.appendChild(input);
+  form.addEventListener('turbo:submit-end', function() { form.remove(); });
+  document.body.appendChild(form);
+  form.requestSubmit();
 }
 window.drop = drop