Sync System Settings tabs with the URL query

activeTab read ?tab= only once at mount, so settings-rail links that
change just the query (Branding, Floor Map) updated the URL without
switching the panel. The query param is now the source of truth: a
watcher applies rail/back/forward navigation and tab clicks write the
query via router.replace.

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

CHANGELOG.md

@@ -76,6 +76,9 @@ ADR-007 and ADR-002.
 
 ### Fixed
 
+- System Settings tabs follow the URL: clicking a settings-rail link that
+  only changes the ?tab= query (Branding, Floor Map) now switches the right
+  panel, tab clicks update the URL, and browser back/forward restore tabs.
 - Following a relationship link between two assets of the same type now loads
   the destination page instead of stale content (router-view keyed on path;
   query-only URL changes still avoid a remount).