feat(customers): sortable table columns + default ascending ID order

- Customer list now defaults to ascending ID order instead of newest-first,
  so the table starts at customer #1 instead of the highest ID
- Add sort_by/sort_order query params to GET /customers (whitelisted column
  map to prevent SQL injection via arbitrary column names)
- Make ID/Name/Subdomain/Status/Devices/Created column headers clickable,
  toggling asc/desc with a visual arrow indicator

Co-Authored-By: Claude Sonnet 5 <[email protected]>

static/css/styles.css

@@ -1,5 +1,25 @@
 /* NetBird MSP Appliance - Custom Styles */
 
+/* Sortable table headers */
+.sortable-th {
+    cursor: pointer;
+    user-select: none;
+    white-space: nowrap;
+}
+
+.sortable-th:hover {
+    color: var(--bs-primary);
+}
+
+.sortable-th .sort-icon {
+    opacity: 0.35;
+}
+
+.sortable-th.sort-asc .sort-icon,
+.sortable-th.sort-desc .sort-icon {
+    opacity: 1;
+}
+
 /* i18n FOUC prevention */
 body.i18n-loading #login-page,
 body.i18n-loading #app-page {