Implement measurements create/destroy and display existing readouts

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

app/views/measurements/index.html.erb

@@ -7,7 +7,18 @@
   <% end %>
 </div>
 
-<table class="main-area">
+<table class="main-area items-table">
+  <thead>
+    <tr>
+      <th><%= Quantity.model_name.human %></th>
+      <th><%= Readout.human_attribute_name(:value) %></th>
+      <th><%= Unit.model_name.human %></th>
+      <th><%= Readout.human_attribute_name(:created_at) %></th>
+      <% if current_user.at_least(:active) %>
+        <th></th>
+      <% end %>
+    </tr>
+  </thead>
   <tbody id="measurements">
     <%= render(@measurements) || render_no_items %>
   </tbody>