Add Plotly line charts view to Measurements page

Users can now switch to a Charts view that renders a separate
time-series line chart for each tracked quantity, using Plotly.js
loaded via CDN. Charts are sorted chronologically and styled to
match the app palette. A dedicated toggle button and matching
CSS visibility rules mirror the existing Compact/Wide view pattern.

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

app/views/layouts/application.html.erb

@@ -14,6 +14,7 @@
     <%= csp_meta_tag %>
 
     <%= stylesheet_link_tag "spreadsheet" %>
+    <script src="https://cdn.plot.ly/plotly-basic-2.35.2.min.js"></script>
     <%= javascript_importmap_tags %>
 
     <%#= turbo_page_requires_reload_tag %>

app/views/measurements/index.html.erb

@@ -11,6 +11,9 @@
   <%= 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">
@@ -34,5 +37,6 @@
   </table>
 
   <div id="measurements-wide" class="measurements-wide"></div>
+  <div id="measurements-charts" class="measurements-charts"></div>
 </div>