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/details_controller.js

@@ -0,0 +1,55 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  static targets = ["countLabel", "submitButton", "list"]
+
+  connect() {
+    this._observer = new MutationObserver(() => {
+      this.element.dispatchEvent(new Event('change', { bubbles: true }))
+    })
+    this._observer.observe(this.listTarget, { subtree: true, attributeFilter: ['disabled'] })
+  }
+
+  disconnect() {
+    this._observer?.disconnect()
+  }
+
+  change() {
+    const count = this.element.querySelectorAll('input:checked:not([disabled])').length
+    if (count > 0) {
+      this.countLabelTarget.textContent = count + ' selected'
+      Turbo.StreamElement.prototype.enableElement(this.submitButtonTarget)
+    } else {
+      this.countLabelTarget.textContent = this.countLabelTarget.dataset.prompt
+      Turbo.StreamElement.prototype.disableElement(this.submitButtonTarget)
+    }
+  }
+
+  close(event) {
+    if (!event.relatedTarget ||
+        event.relatedTarget.closest("details") != this.element) {
+      this.element.removeAttribute("open")
+    }
+  }
+
+  processKey(event) {
+    switch (event.key) {
+      case "Escape":
+        if (this.element.hasAttribute("open")) {
+          this.element.removeAttribute("open")
+          event.stopPropagation()
+        }
+        break
+      case "Enter": {
+        const button = this.element.querySelector("button:not([disabled])")
+        if (button) {
+          button.click()
+          event.target.blur()
+          event.preventDefault()
+          event.stopPropagation()
+        }
+        break
+      }
+    }
+  }
+}