CVE-2026-27877: fix exposure of direct data-source passwords via public
dashboards by limiting frontend settings to data sources actually used by
the dashboard
Note: upstream test additions in pkg/api/frontendsettings_test.go are not
backported. The %check stage only runs the Jest frontend suite (gated on
0), so backend Go tests would not be exercised by this
build, and the upstream test depends on hs.publicDashboardsService which
does not exist in 10.2.6 (the production fix already uses the
hs.PublicDashboardsApi.PublicDashboardService adapter for the same reason)