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/CONFIG.md

@@ -166,7 +166,7 @@ them under `instance/branding/`.
 
 | Key | Default | Notes |
 |-----|---------|-------|
-| `map_blueprint_light` | `/static/images/floorplan-placeholder.svg` | Floor-map blueprint (light theme). Re-upload your own in Settings > Map. |
+| `map_blueprint_light` | `/static/images/floorplan-placeholder.svg` | Floor-map blueprint (light theme). Re-upload your own in Settings > Floor Map. |
 | `map_blueprint_dark` | `/static/images/floorplan-placeholder.svg` | Floor-map blueprint (dark theme). |
 | `map_width` | `3300` | Blueprint native width in pixels. |
 | `map_height` | `2550` | Blueprint native height in pixels. |