Refactor charts: dedicated nav tab, JSON data transport, tests

Replace the toggle-view approach and hidden DOM data carrier with a
proper dedicated Charts page:

- Move Charts out of Measurements view toggles into its own nav tab
  and route (GET /charts)
- ChartsController serializes readout data as JSON (ordered by
  taken_at); the view embeds it in a <script type="application/json">
  element instead of rendering a hidden copy of the measurements
  partial just to ferry data attributes to JS
- buildCharts() reads from the JSON element directly — no DOM parsing,
  no sorting in JS (server already orders the data)
- Turbo load handler detects the charts page via #charts-data presence
- Add controller tests (authentication, data shape, ordering,
  data isolation between users) and system tests

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

app/views/measurements/index.html.erb

@@ -11,9 +11,7 @@
   <%= image_button_tag '', 'view-columns', name: nil, type: 'button',
     class: 'view-toggle', title: t('.view_wide'),
     data: {view: 'wide'}, onclick: "setMeasurementsView('wide')" %>
-  <%= image_button_tag '', 'chart-line', name: nil, type: 'button',
-    class: 'view-toggle', title: t('.view_charts'),
-    data: {view: 'charts'}, onclick: "setMeasurementsView('charts')" %>
+
 </div>
 
 <div class="main-area measurements-section">
@@ -37,6 +35,6 @@
   </table>
 
   <div id="measurements-wide" class="measurements-wide"></div>
-  <div id="measurements-charts" class="measurements-charts"></div>
+
 </div>