feat: per-group sort order (defined/alpha) in group form and converter

src/routes/admin/+page.svelte

@@ -803,6 +803,15 @@
         <small class="muted">Relative scale for cross-group (YOLO) conversions. Arbitrary.</small>
         <input type="number" bind:value={groupForm.toUniversal} min="0.000001" step="any" />
       </label>
+      <label>
+        Display order
+        <small class="muted">How units in this group are sorted in the converter.</small>
+        <select bind:value={groupForm.sortOrder}>
+          <option value={undefined}>Defined order (default)</option>
+          <option value="defined">Defined order</option>
+          <option value="alpha">Alphabetical</option>
+        </select>
+      </label>
       <div class="form-actions">
         <button onclick={saveGroup}>{groupFormNew ? 'Create' : 'Save'}</button>
         {#if !groupFormNew}