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/charts/index.html.erb

@@ -0,0 +1,2 @@
+<div class="main-area" id="measurements-charts"></div>
+<script id="charts-data" type="application/json"><%= raw @readouts_json %></script>