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>

config/environments/test.rb

@@ -58,4 +58,7 @@ Rails.application.configure do
   # config.action_view.annotate_rendered_view_with_filenames = true
 
   config.log_level = :info
+
+  # Allow Capybara's dynamic test server host (127.0.0.1:<random_port>)
+  config.hosts << '127.0.0.1'
 end

config/locales/en.yml

@@ -11,8 +11,13 @@ en:
   activerecord:
     attributes:
       quantity:
+        default_unit: Default unit
         description: Description
         name: Name
+      readout:
+        created_at: Recorded at
+        taken_at: Taken at
+        value: Value
       unit:
         base: Base unit
         description: Description
@@ -81,6 +86,9 @@ en:
       revert: Revert
       sign_out: Sign out
       source_code: Get code
+  readouts:
+    form:
+      set_default_unit: Set as default unit
   measurements:
     navigation: Measurements
     no_items: There are no measurements taken. You can Add some now.
@@ -89,6 +97,15 @@ en:
       taken_at_html: Measurement taken at 
     index:
       new_measurement: Add measurement
+    readout:
+      destroy: Delete
+    create:
+      success:
+        one: Recorded 1 measurement.
+        other: Recorded %{count} measurements.
+      no_readouts: No readouts selected.
+    destroy:
+      success: Measurement deleted.
   quantities:
     navigation: Quantities
     no_items: There are no configured quantities. You can Add some or Import from defaults.