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>

CHANGELOG.md

@@ -92,6 +92,20 @@ ADR-007 and ADR-002.
     no-target list with dispositions, a worked idempotent Python importer, and
     row-count parity checks.
 
+### Changed
+
+- System Settings is no longer one tabbed page. The inner tab bar is gone and
+  each section is its own routed settings page reached through the settings
+  rail: ServiceNow, Zabbix Supplies, Dell Warranty, and Collector PC Types
+  (the former Integrations dumping ground, now split three-plus ways), plus
+  Branding, Floor Map, Printing & Labels, Email / SMTP, Authentication,
+  Audit & Logging, Asset Identifiers, and Global Search. The rail regroups
+  these under Site & Facility, Integrations, Communication, Search & Identity,
+  and Access & Security. Shared load/save/upload plumbing moved into a
+  `useSystemSettings` composable so the pages stay thin. Old bookmarks keep
+  working: `/settings/system` and every `/settings/system?tab=<key>` redirect
+  to the matching new page.
+
 ### Fixed
 
 - Site & Facility settings page renders booleans as toggles and the