Dissolve System Settings into individual settings pages

The monolithic tab page competed with the settings rail as a second
navigation system, and its Integrations tab was a dumping ground. Each
section is now its own routed rail page (ServiceNow, Zabbix Supplies,
Dell Warranty, Collector PC Types, Branding, Floor Map, Printing and
Labels, Email/SMTP, Audit, Authentication, Asset Identifiers, Global
Search), thin over a shared useSystemSettings composable, grouped
logically in the rail with system groups clustered last. Old
/settings/system?tab= URLs redirect to the right page.

Also fixes the post-login redirect: the auth guard now remembers the
intended destination and Login returns there (same-site paths only).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/router/index.js

@@ -105,7 +105,8 @@ router.beforeEach(async (to, from, next) => {
   const authStore = useAuthStore()
 
   if (to.meta.requiresAuth && !authStore.isAuthenticated) {
-    return next('/login')
+    // Remember where they were headed so login can send them back there.
+    return next({ path: '/login', query: { redirect: to.fullPath } })
   }
   if (to.meta.requiresAdmin && !authStore.isAdmin) {
     return next('/')