Make the explore dashboard filter bar sticky

On explore-level analytics dashboards, the scope and date range filters scroll out of view on tall dashboards, forcing users back to the top to change them. This keeps the filter bar (view tabs, scope, date range, reset, and EE filter actions) stuck below the application header while content scrolls, reusing the Security Dashboard sticky pattern. Sticky applies from the md breakpoint up; below that the vertically stacked bar would cover most of a small viewport.

Two deliberate scope calls: below the md breakpoint the bar scrolls normally, since the vertically stacked filters would cover most of a small viewport (UX ack welcome), and the change is pure CSS with no feature flag of its own, so rollback is an MR revert. Group and project dashboard routes ride the existing consolidate_analytics_dashboards flag.

Filter changes reload panels in place (no dashboard teardown), so the page does not jump to the top; a new spec pins this.

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/629213

Screen Recording:

How to validate

1. Enable the explore_analytics_dashboards feature flag, open /explore/analytics_dashboards/<slug>, pick a scope, scroll down: the filter bar stays below the header with an opaque background.
2. Change the date range while scrolled: panels reload in place, no jump to top.
3. Narrow the window below 768px: the bar scrolls normally.