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/javascript/application.js

@@ -47,10 +47,8 @@ function buildWideTable() {
     groupMap.get(key).rows.push(r);
   });
 
-  // Read column headers from compact thead
-  var ths = document.querySelectorAll('.measurements-compact thead th');
-  var takenAtHeader = ths[3] ? ths[3].textContent : '';
-  var createdAtHeader = ths[4] ? ths[4].textContent : '';
+  var takenAtHeader = (document.querySelector('[data-column="taken-at"]') || {}).textContent || '';
+  var createdAtHeader = (document.querySelector('[data-column="created-at"]') || {}).textContent || '';
 
   var table = document.createElement('table');
   table.className = 'items-table';
@@ -190,7 +188,8 @@ function editMeasurementWide(url) {
         panel.scrollIntoView({behavior: 'smooth', block: 'nearest'});
       }
     });
-  });
+  })
+  .catch(err => console.error('editMeasurementWide failed:', err));
 }
 window.editMeasurementWide = editMeasurementWide
 
@@ -229,7 +228,8 @@ function setDefaultUnit(button) {
     }
     return response.text();
   })
-  .then(html => Turbo.renderStreamMessage(html));
+  .then(html => Turbo.renderStreamMessage(html))
+  .catch(err => console.error('setDefaultUnit failed:', err));
 }
 window.setDefaultUnit = setDefaultUnit