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/views/settings/EmployeeDirectory.vue

@@ -13,7 +13,7 @@
         The directory is in <strong>external</strong> mode - people come from a
         separate HR database, so there is nothing to manage here. To manage
         people in-app, set <code>employee_directory_mode</code> to
-        <code>selfhosted</code> under System Settings, then reload.
+        <code>selfhosted</code> under Site &amp; Facility settings, then reload.
       </p>
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