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>

docs/UPGRADE.md

@@ -97,7 +97,7 @@ removes those bundled PNGs and ships a generic placeholder SVG instead.
 
 If your instance's `map_blueprint_light` / `map_blueprint_dark` Settings still
 point at `/static/images/sitemap2025-*`, the map will 404 those images after the
-upgrade. Re-upload your own floor plan in **Settings > Map**. Uploaded floor
+upgrade. Re-upload your own floor plan in **Settings > Floor Map**. Uploaded floor
 plans are stored under `instance/` and survive upgrades, so a site that already
 uploaded its own plan is unaffected. Only instances still using the old bundled
 default need to act.