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/views/quantities/_form.html.erb

@@ -8,6 +8,11 @@
     <td>
       <%= form.text_area :description, cols: 30, rows: 1, escape: false %>
     </td>
+    <td>
+      <%= form.collection_select :default_unit_id, @user_units, :id,
+        ->(u){ sanitize('&emsp;' * (u.base_id? ? 1 : 0) + u.symbol) },
+        {include_blank: true}, onchange: "this.dataset.changed = ''" %>
+    </td>
 
     <td class="flex">
       <%= form.button %>