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/users_test.rb

@@ -229,7 +229,7 @@ class UsersTest < ApplicationSystemTestCase
       user = User.find_by_email!(first(:link).text)
       inject_button_to first('td:not(.link)'), "update status", user_path(user), method: :patch,
         params: {user: {status: User.statuses.keys.sample}}, data: {turbo: false}
-      click_on "update status"
+      execute_script("arguments[0].click()", find_button("update status"))
     end
     assert_title 'The change you wanted was rejected (422)'
   end