feat(frontend): reset scroll to top on every route navigation

Navigating between pages (e.g. Home -> Privacy Policy) could leave the
user partway down the new page, because the browser kept the previous
page's scroll position.

Reset the scroll to the top from the CSSTransition `onEnter`, i.e. as the
incoming page begins entering. With the out-in transition the old page
fades out at its position, then the new page mounts at the top and fades
in -- so there's no jarring mid-scroll jump; the existing fade/slide is
the smoothness. Tied to the keyed transition (location.pathname), so
in-page hash/anchor links are unaffected.

60 tests / 10 snapshots pass; eslint + prettier clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hoangsonww

frontend/src/App.js

@@ -77,6 +77,18 @@ function AppLayout() {
     window.matchMedia("(prefers-reduced-motion: reduce)").matches;
   const timeout = reduceMotion ? 0 : { enter: 480, exit: 300 };
 
+  // Reset scroll to the top as the new page begins entering. With the
+  // out-in transition the old page fades out at its scroll position, then
+  // the incoming page mounts here at the top and fades in -- so navigating
+  // (e.g. Home -> Privacy Policy) never drops the user mid-page, and the
+  // existing fade/slide provides the smoothness rather than a jarring
+  // mid-scroll jump. Tied to the keyed CSSTransition (location.pathname),
+  // so in-page hash/anchor links are left alone.
+  const scrollToTopOnEnter = () => {
+    if (typeof window === "undefined") return;
+    window.scrollTo(0, 0);
+  };
+
   return (
     <>
       {!hideNavbar && <Navbar />}
@@ -88,6 +100,7 @@ function AppLayout() {
           classNames="page"
           appear
           unmountOnExit
+          onEnter={scrollToTopOnEnter}
         >
           <div ref={nodeRef} className="page-anim">
             <Routes location={location}>