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>

server/routes.js

@@ -69,8 +69,8 @@ router.get('/instances/stacks', (_req, res) => {
 
 // GET /api/instances
 router.get('/instances', (req, res) => {
-  const { search, state, stack } = req.query;
-  res.json(getInstances({ search, state, stack }));
+  const { search, state, stack, sort, order } = req.query;
+  res.json(getInstances({ search, state, stack, sort, order }));
 });
 
 // GET /api/instances/:vmid/history