Settings nav items are plain Turbo Drive links (full-body visits); Turbo only
restores window scroll, so the nested overflow-y-auto content container snapped
to the top on every settings navigation.
Add a settings-scroll Stimulus controller that saves/restores the content scroll
keyed by pathname: returning to a page restores its scroll, a new page opens at
the top, and a same-page re-render (settings form auto-submit) keeps scroll.
Distinct from the nav's preserve-scroll controller, which keys by element id to
intentionally carry one position across pages.
Verified live: scroll 250 on Preferences -> Appearance opens at top -> back to
Preferences restores 250.
