Migrate all inline JS to Stimulus controllers

Add stimulus-rails gem and wire up 7 controllers:
- measurements_view_controller: view toggle (compact/wide) via localStorage
- measurements_controller: grouped rows MutationObserver
- charts_controller: Plotly chart rendering
- form_controller: keyboard shortcuts (Escape/Enter) and submit validation
- details_controller: quantity picker state, focusout close, MutationObserver
- readout_unit_controller: default unit button enable/disable + PATCH submission
- drag_controller: drag-and-drop for quantity reparenting and unit rebasing

Remove all inline onclick/onkeydown/ondrag*/onsubmit handlers from templates.
Remove all window.* global exports from application.js.
Remove bare <script> block from measurements/_form.html.erb.
Remove turbo:load listeners for behavior now in controller connect().

application.js now only contains: Turbo Stream custom action definitions
and the showPage visibility listener.

Document Stimulus conventions in CLAUDE.md.

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

app/javascript/controllers/readout_unit_controller.js

@@ -0,0 +1,36 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  static targets = ["select", "button"]
+
+  unitChanged() {
+    if (this.selectTarget.value && this.selectTarget.value !== this.selectTarget.dataset.defaultUnitId) {
+      Turbo.StreamElement.prototype.enableElement(this.buttonTarget)
+    } else {
+      Turbo.StreamElement.prototype.disableElement(this.buttonTarget)
+    }
+  }
+
+  setDefault() {
+    const select = this.selectTarget
+    const form = document.createElement('form')
+    form.action = this.buttonTarget.dataset.path
+    form.method = 'post'
+    form.dataset.turboStream = 'true'
+    const methodInput = document.createElement('input')
+    methodInput.type = 'hidden'; methodInput.name = '_method'; methodInput.value = 'patch'
+    const 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', event => {
+      if (event.detail.success) {
+        select.dataset.defaultUnitId = select.value
+        this.unitChanged()
+      }
+      form.remove()
+    })
+    document.body.appendChild(form)
+    form.requestSubmit()
+  }
+}