Fix column header lookup fragility, add fetch error handling, add tests

- Replace position-based column header lookup (ths[3]/ths[4]) with
  data-column attribute selectors — immune to column reordering
- Add .catch() error handlers to editMeasurementWide and setDefaultUnit
  fetch calls so failures surface in the console instead of silently
  disappearing
- Add MeasurementsController integration tests covering index auth,
  create with taken_at, empty-readout create, destroy, cross-user
  destroy isolation, and update

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

app/views/measurements/index.html.erb

@@ -21,8 +21,8 @@
         <th><%= Quantity.model_name.human %></th>
         <th><%= Readout.human_attribute_name(:value) %></th>
         <th><%= Unit.model_name.human %></th>
-        <th><%= Readout.human_attribute_name(:taken_at) %></th>
-        <th><%= Readout.human_attribute_name(:created_at) %></th>
+        <th data-column="taken-at"><%= Readout.human_attribute_name(:taken_at) %></th>
+        <th data-column="created-at"><%= Readout.human_attribute_name(:created_at) %></th>
         <% if current_user.at_least(:active) %>
           <th></th>
         <% end %>