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>

test/controllers/charts_controller_test.rb

@@ -0,0 +1,63 @@
+require "test_helper"
+
+class ChartsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    host! '127.0.0.1'
+    @user = users(:alice)
+    post new_user_session_path, params: { user: { email: @user.email, password: 'alice' } }
+    @quantity = @user.quantities.create!(name: 'Weight')
+    @unit = @user.units.create!(symbol: 'kg')
+  end
+
+  test "requires authentication" do
+    delete destroy_user_session_path
+    get charts_path
+    assert_response :redirect
+  end
+
+  test "index returns ok" do
+    get charts_path
+    assert_response :success
+  end
+
+  test "embeds readout data as JSON in script tag" do
+    users(:alice).readouts.create!(quantity: @quantity, unit: @unit, value: 82.5, taken_at: 1.day.ago)
+
+    get charts_path
+
+    assert_select 'script#charts-data[type="application/json"]' do |elements|
+      data = JSON.parse(elements.first.children.first.to_s)
+      assert_equal 1, data.size
+      assert_equal 'Weight', data.first['quantityName']
+      assert_in_delta 82.5, data.first['value']
+      assert_equal 'kg', data.first['unit']
+      assert_not_nil data.first['takenAt']
+    end
+  end
+
+  test "orders readouts by taken_at ascending" do
+    older = users(:alice).readouts.create!(quantity: @quantity, unit: @unit, value: 80.0, taken_at: 2.days.ago)
+    newer = users(:alice).readouts.create!(quantity: @quantity, unit: @unit, value: 82.5, taken_at: 1.day.ago)
+
+    get charts_path
+
+    assert_select 'script#charts-data[type="application/json"]' do |elements|
+      data = JSON.parse(elements.first.children.first.to_s)
+      assert_equal older.taken_at.iso8601, data.first['takenAt']
+      assert_equal newer.taken_at.iso8601, data.last['takenAt']
+    end
+  end
+
+  test "does not expose other users readouts" do
+    bob_quantity = users(:bob).quantities.create!(name: 'Steps')
+    bob_unit = users(:bob).units.create!(symbol: 'steps')
+    users(:bob).readouts.create!(quantity: bob_quantity, unit: bob_unit, value: 5000, taken_at: 1.day.ago)
+
+    get charts_path
+
+    assert_select 'script#charts-data[type="application/json"]' do |elements|
+      data = JSON.parse(elements.first.children.first.to_s)
+      assert data.none? { |r| r['quantityName'] == 'Steps' }, "Bob's data must not appear"
+    end
+  end
+end

test/system/charts_test.rb

@@ -0,0 +1,26 @@
+require "application_system_test_case"
+
+class ChartsTest < ApplicationSystemTestCase
+  setup do
+    @user = sign_in(user: users(:alice))
+    @quantity = @user.quantities.create!(name: 'Weight')
+    @unit = @user.units.create!(symbol: 'kg')
+    @user.readouts.create!(quantity: @quantity, unit: @unit, value: 82.5, taken_at: 1.day.ago)
+    @user.readouts.create!(quantity: @quantity, unit: @unit, value: 83.1, taken_at: Time.now)
+    visit charts_path
+  end
+
+  test "charts page is reachable from navigation" do
+    visit root_path
+    click_on t('charts.navigation')
+    assert_current_path charts_path
+  end
+
+  test "renders Plotly chart panel" do
+    assert_selector '#measurements-charts .chart-panel', wait: 5
+  end
+
+  test "chart legend shows quantity name with unit" do
+    assert_text 'Weight (kg)', wait: 5
+  end
+end