fix(settings): preserve content scroll position per page across navigation (#2277)

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.

app/javascript/controllers/settings_scroll_controller.js

@@ -0,0 +1,39 @@
+import { Controller } from "@hotwired/stimulus";
+
+// Preserves the settings content scroll position PER PAGE across Turbo Drive
+// navigation. Settings nav items are plain links (full-body Turbo visits), and
+// Turbo only restores window scroll — so this nested overflow-y-auto container
+// snaps to top on every visit.
+//
+// Keyed by pathname: returning to a page restores its scroll, a brand-new page
+// starts at the top, and a same-page re-render (e.g. a settings form that
+// auto-submits) keeps scroll. This differs from the nav's `preserve-scroll`
+// controller, which intentionally carries one position across all pages because
+// the nav is the same persistent element on every settings page.
+export default class extends Controller {
+  static positions = {};
+
+  connect() {
+    this.save = this.save.bind(this);
+    document.addEventListener("turbo:before-cache", this.save);
+    this.restore();
+  }
+
+  disconnect() {
+    document.removeEventListener("turbo:before-cache", this.save);
+  }
+
+  save() {
+    this.constructor.positions[window.location.pathname] = {
+      top: this.element.scrollTop,
+      left: this.element.scrollLeft,
+    };
+  }
+
+  restore() {
+    const pos = this.constructor.positions[window.location.pathname];
+    if (!pos) return;
+    this.element.scrollTop = pos.top;
+    this.element.scrollLeft = pos.left;
+  }
+}