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/assets/stylesheets/application.css

@@ -661,29 +661,18 @@ li::marker {
   overflow-x: auto;
 }
 body[data-measurements-view=wide] .measurements-compact,
-body[data-measurements-view=wide] .measurements-charts,
-body[data-measurements-view=compact] .measurements-wide,
-body[data-measurements-view=compact] .measurements-charts,
-body[data-measurements-view=charts] .measurements-compact,
-body[data-measurements-view=charts] .measurements-wide {
+body[data-measurements-view=compact] .measurements-wide {
   display: none;
 }
 body[data-measurements-view=compact] .view-toggle[data-view=compact],
-body[data-measurements-view=wide] .view-toggle[data-view=wide],
-body[data-measurements-view=charts] .view-toggle[data-view=charts] {
+body[data-measurements-view=wide] .view-toggle[data-view=wide] {
   background-color: var(--color-blue);
   border-color: var(--color-blue);
   color: white;
   fill: white;
 }
-.measurements-charts {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 1em;
-}
 .chart-panel {
-  flex: 1 1 400px;
-  min-width: 300px;
+  width: 100%;
 }
 #measurements tr.grouped td {
   border-top: none;