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>

app/controllers/quantities_controller.rb

@@ -8,6 +8,10 @@ class QuantitiesController < ApplicationController
     raise AccessForbidden unless current_user.at_least(:active)
   end
 
+  before_action only: [:new, :edit, :create, :update] do
+    @user_units = current_user.units.ordered
+  end
+
   def index
     @quantities = current_user.quantities.ordered.includes(:parent, :subquantities)
   end