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/charts_controller.js

@@ -0,0 +1,45 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  static targets = ["container", "data"]
+
+  connect() {
+    const readouts = JSON.parse(this.dataTarget.textContent)
+    if (readouts.length === 0) return
+
+    const quantities = new Map()
+    readouts.forEach(r => {
+      if (!r.takenAt) return
+      if (!quantities.has(r.quantityId)) {
+        quantities.set(r.quantityId, { name: r.quantityName, unit: r.unit, x: [], y: [] })
+      }
+      const q = quantities.get(r.quantityId)
+      q.x.push(r.takenAt)
+      q.y.push(r.value)
+    })
+
+    const traces = []
+    quantities.forEach(q => {
+      traces.push({
+        x: q.x, y: q.y,
+        mode: 'lines+markers', type: 'scatter',
+        name: q.name + ' (' + q.unit + ')',
+        marker: { size: 5 }
+      })
+    })
+
+    const div = document.createElement('div')
+    div.className = 'chart-panel'
+    this.containerTarget.appendChild(div)
+
+    Plotly.newPlot(div, traces, {
+      xaxis: { type: 'date', tickformat: '%Y-%m-%d %H:%M' },
+      yaxis: {},
+      margin: { t: 20, r: 20, b: 80, l: 60 },
+      paper_bgcolor: 'transparent',
+      plot_bgcolor: 'transparent',
+      font: { family: 'system-ui' },
+      legend: { orientation: 'h', y: -0.25 }
+    }, { responsive: true, displayModeBar: false })
+  }
+}