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>

js/ui.js

@@ -100,11 +100,21 @@ function setStateFilter(state) {
   filterInstances();
 }
 
+function toggleSortDir() {
+  const btn = document.getElementById('sort-dir');
+  const next = btn.dataset.dir === 'asc' ? 'desc' : 'asc';
+  btn.dataset.dir = next;
+  btn.textContent = next === 'asc' ? '↑' : '↓';
+  filterInstances();
+}
+
 async function filterInstances() {
   const search = document.getElementById('search-input').value;
   const state  = document.getElementById('filter-state').value;
   const stack  = document.getElementById('filter-stack').value;
-  const instances = await getInstances({ search, state, stack });
+  const sort   = document.getElementById('sort-field').value;
+  const order  = document.getElementById('sort-dir').dataset.dir || 'asc';
+  const instances = await getInstances({ search, state, stack, sort, order });
   const grid = document.getElementById('instance-grid');
 
   if (!instances.length) {