feat: add sort by vmid, name, last created, last updated on dashboard

- GET /api/instances now accepts ?sort= (name|vmid|created_at|updated_at)
  and ?order= (asc|desc); invalid sort fields fall back to name asc
- Timestamp sorts use id as a tiebreaker (datetime() precision is 1 s)
- Toolbar gains a sort-field <select> and a ↑/↓ direction toggle button
- toggleSortDir() flips direction and re-fetches; state held in data-dir attr

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

index.html

@@ -48,6 +48,13 @@
         <select id="filter-stack" onchange="filterInstances()">
           <option value="">all stacks</option>
         </select>
+        <select id="sort-field" onchange="filterInstances()">
+          <option value="name">name</option>
+          <option value="vmid">vmid</option>
+          <option value="updated_at">last updated</option>
+          <option value="created_at">last created</option>
+        </select>
+        <button id="sort-dir" class="btn" data-dir="asc" onclick="toggleSortDir()" title="reverse sort">↑</button>
         <div class="toolbar-right">
           <button class="btn primary" onclick="openNewModal()">+ new instance</button>
         </div>