Improve index tables

Stretch inputs to column width in place of fixed sizing.
Set minimum column widths for columns with form inputs.
Simplify table operations by removing `thead` and adding sentinel.
Manage empty table display with CSS only.

app/views/units/_form.html.erb

@@ -3,20 +3,17 @@
     data: {link: link, form: form_tag, hidden_row: hidden_row} do %>
 
     <td style="--depth:<%= @unit.base_id? ? 1 : 0 %>">
-      <%= form.text_field :symbol, required: true, autofocus: true, size: 12 %>
-    </td>
+      <%= form.text_field :symbol, autofocus: true, class: 'hstretch', required: true %>
     <td>
-      <%= form.text_area :description, cols: 30, rows: 1, escape: false %>
-    </td>
-    <td>
-      <%= form.number_field :multiplier, required: true if @unit.base_id? %>
-    </td>
+      <% if @unit.base_id? %>
+        <%= form.number_field :multiplier, class: 'hstretch', required: true %>
+      <% end %>
+    <td><%= form.text_area :description, class: 'hstretch', escape: false, rows: 1 %>
 
     <td class="flex">
       <%= form.button %>
       <%= image_link_to t(:cancel), "close-outline", units_path, class: 'dangerous',
         name: :cancel, onclick: render_turbo_stream('form_close', {row: row}) %>
-    </td>
-    <td></td>
+    <td>
   <% end %>
 <% end %>