fix(motion): respect prefers-reduced-motion across smooth scrolls

Programmatic `scrollIntoView` / `scrollTo` calls in the settings shell,
admin shell, and coach message thread ignored the user's reduced-motion
preference — a WCAG 2.3.3 violation. Add a single `scrollBehaviorForUser`
helper in `src/lib/motion.ts` that returns `"auto"` when SSR or when
`(prefers-reduced-motion: reduce)` matches, and `"smooth"` otherwise.
Wire it through the three call-sites; tests cover all three branches.

src/components/admin/admin-shell.tsx

@@ -35,6 +35,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { scrollBehaviorForUser } from "@/lib/motion";
 import { useTranslations } from "@/lib/i18n/context";
 import { isAdminSectionSlug, type AdminSectionSlug } from "./section-slugs";
 
@@ -183,7 +184,8 @@ export function AdminShell({ active, children }: AdminShellProps) {
       // and the first one or two chips were unreachable on narrow
       // viewports.
       inline: "start",
-      behavior: "smooth",
+      // v1.4.43 W5-H5 — respect `prefers-reduced-motion`.
+      behavior: scrollBehaviorForUser(),
     });
   }, [activeSlug]);
 

src/components/insights/coach-panel/message-thread.tsx

@@ -12,6 +12,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { scrollBehaviorForUser } from "@/lib/motion";
 import { useTranslations } from "@/lib/i18n/context";
 import { stripChartTokens } from "@/lib/insights/chart-tokens";
 import { useAuth } from "@/hooks/use-auth";
@@ -194,7 +195,8 @@ export function MessageThread({
     const el = scrollerRef.current;
     if (!el) return;
     if (wasPinnedRef.current) {
-      el.scrollTo({ top: el.scrollHeight, behavior: "smooth" });
+      // v1.4.43 W5-H5 — respect `prefers-reduced-motion`.
+      el.scrollTo({ top: el.scrollHeight, behavior: scrollBehaviorForUser() });
     }
   }, [messages.length, streaming?.content, optimisticUser?.localId]);
 

src/components/settings/settings-shell.tsx

@@ -31,6 +31,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { scrollBehaviorForUser } from "@/lib/motion";
 import { useTranslations } from "@/lib/i18n/context";
 import {
   SETTINGS_SECTION_SLUGS,
@@ -169,7 +170,8 @@ export function SettingsShell({ active, children }: SettingsShellProps) {
       // and the first one or two chips were unreachable on narrow
       // viewports.
       inline: "start",
-      behavior: "smooth",
+      // v1.4.43 W5-H5 — respect `prefers-reduced-motion`.
+      behavior: scrollBehaviorForUser(),
     });
   }, [activeSlug]);
 

src/lib/__tests__/motion.test.ts

@@ -0,0 +1,44 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { scrollBehaviorForUser } from "@/lib/motion";
+
+// The vitest config runs unit tests under Node (no jsdom). The helper
+// only touches `window.matchMedia`, so we mount a minimal stub via
+// `vi.stubGlobal` and tear it down after each case.
+
+function mountWindow(matchesReducedMotion: boolean) {
+  vi.stubGlobal("window", {
+    matchMedia: (query: string) => ({
+      matches:
+        matchesReducedMotion && query === "(prefers-reduced-motion: reduce)",
+      media: query,
+      onchange: null,
+      addListener: () => {},
+      removeListener: () => {},
+      addEventListener: () => {},
+      removeEventListener: () => {},
+      dispatchEvent: () => false,
+    }),
+  });
+}
+
+describe("scrollBehaviorForUser", () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("returns `auto` when window is undefined (SSR)", () => {
+    vi.stubGlobal("window", undefined);
+    expect(scrollBehaviorForUser()).toBe("auto");
+  });
+
+  it("returns `auto` when prefers-reduced-motion matches", () => {
+    mountWindow(true);
+    expect(scrollBehaviorForUser()).toBe("auto");
+  });
+
+  it("returns `smooth` when prefers-reduced-motion does not match", () => {
+    mountWindow(false);
+    expect(scrollBehaviorForUser()).toBe("smooth");
+  });
+});

src/lib/motion.ts

@@ -0,0 +1,13 @@
+// v1.4.43 W5-H5 — single source of truth for `behavior` on the four
+// programmatic `scrollIntoView` / `scrollTo` call-sites in the app.
+// `prefers-reduced-motion: reduce` users were previously animated against
+// their OS-level preference, which is a WCAG 2.3.3 violation. The helper
+// returns `"auto"` whenever a window is unavailable (SSR) or when the
+// reduced-motion media query matches.
+
+export function scrollBehaviorForUser(): ScrollBehavior {
+  if (typeof window === "undefined") return "auto";
+  return window.matchMedia("(prefers-reduced-motion: reduce)").matches
+    ? "auto"
+    : "smooth";
+}