Add taken_at to readouts and default unit to quantities

Readouts gain a taken_at timestamp (distinct from created_at) that records
when the measurement was actually taken. Measurements are now ordered by
taken_at descending.

Quantities gain an optional default_unit association. When set, the unit
is pre-selected in the measurement form. A "Set as default" button on the
unit selector lets users update the default directly from the form.

- Migrations: add taken_at (datetime) to readouts,
              add default_unit_id (fk → units) to quantities
- Readout: expose taken_at in ATTRIBUTES permit-list
- Quantity: add default_unit belongs_to, expose in ATTRIBUTES
- QuantitiesController: load @user_units for form actions
- Quantities views: add Default unit column and select to form
- Readouts form: pre-select default unit; add "Set as default" button
  (readoutUnitChanged / setDefaultUnit wired up in a later commit)
- Measurements form: default taken_at input to current time
- ApplicationHelper: propagate :form option to html_options in builder
- config/environments/test.rb: allow Capybara's dynamic host
- Tests: system tests for default-unit UI on the Quantities page

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

test/system/quantities_test.rb

@@ -0,0 +1,45 @@
+require "application_system_test_case"
+
+class QuantitiesTest < ApplicationSystemTestCase
+  setup do
+    @user = sign_in(user: users(:alice))
+    @unit = @user.units.create!(symbol: 'kg')
+    @quantity = @user.quantities.create!(name: 'Weight')
+    visit quantities_path
+  end
+
+  test "update button turns red when default unit changes" do
+    click_on 'Weight'
+
+    button = find('button[name=button]')
+    initial_color = evaluate_script("getComputedStyle(arguments[0]).backgroundColor", button)
+
+    select 'kg', from: 'quantity[default_unit_id]'
+
+    changed_color = evaluate_script("getComputedStyle(arguments[0]).backgroundColor", button)
+    refute_equal initial_color, changed_color, "Button color should change when default unit is altered"
+  end
+
+  test "saving default unit pre-selects it in measurements form" do
+    click_on 'Weight'
+    select 'kg', from: 'quantity[default_unit_id]'
+    click_on t('helpers.submit.update')
+    assert_selector '.flash.notice'
+
+    @quantity.reload
+    assert_equal @unit.id, @quantity.default_unit_id
+
+    visit measurements_path
+    find(:link_or_button, t('measurements.index.new_measurement')).click
+    assert_selector '#measurement_form'
+
+    within '#quantity_select' do
+      check 'Weight'
+    end
+    find('button[formaction]').click
+
+    within 'tbody#readouts' do
+      assert_selector "option[value='#{@unit.id}'][selected]"
+    end
+  end
+end