Replace fetch() calls with Turbo form submission via requestSubmit()

setDefaultUnit and drop previously made raw fetch() requests and called
Turbo.renderStreamMessage() manually. Now both create a temporary <form>,
append hidden inputs, and call form.requestSubmit() — Turbo intercepts
the submission natively, handling CSRF, stream responses, and lifecycle.

Also document this convention in CLAUDE.md.

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

CLAUDE.md

@@ -77,6 +77,28 @@ default/       namespace for default units import/export and admin panel
 root           → /units (authenticated), /sign_in (unauthenticated)
 ```
 
+## JavaScript Conventions
+
+### No manual fetch() — use Turbo
+
+Never make AJAX requests with `fetch()` in JavaScript. Use Turbo's built-in mechanisms instead:
+
+- **Links/buttons that trigger server actions**: use `data: {turbo_stream: true}` on the element (link or button_to form).
+- **Dynamic form submissions from JS** (where HTML alone isn't enough): create a form element, append hidden inputs, and call `form.requestSubmit()`. Turbo intercepts it automatically — no manual CSRF handling, no `Turbo.renderStreamMessage()`.
+  ```javascript
+  var form = document.createElement('form');
+  form.action = url; form.method = 'post'; form.dataset.turboStream = 'true';
+  // append hidden inputs...
+  form.addEventListener('turbo:submit-end', function() { form.remove(); });
+  document.body.appendChild(form);
+  form.requestSubmit();
+  ```
+- **Server-rendered HTML**: use ERB partials and Turbo Stream views (`*.turbo_stream.erb`), never build HTML in JavaScript.
+
+### No HTML generation in JavaScript
+
+Never use JavaScript to build and insert HTML (no `innerHTML =`, no `createElement` trees for content). Render HTML server-side in ERB partials; update the DOM via Turbo Stream actions (`replace`, `update`, `append`, etc.).
+
 ## Database Requirements
 
 The database must support:

app/javascript/application.js

@@ -147,27 +147,25 @@ 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
 
@@ -340,22 +338,17 @@ 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