fix: use opacity-only fade for page transitions instead of fade-in-up

ViewTransitionOutlet was using animate-fade-in-up (opacity + translateY)
on every route change, causing visible content shift on navigation.
Switch to animate-fade-in (opacity only) for a subtler transition.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/ViewTransitionOutlet.tsx

@@ -19,7 +19,7 @@ export function ViewTransitionOutlet() {
   const routeKey = segments.slice(0, 2).join('/') || '/'
 
   return (
-    <div key={routeKey} className="flex-1 min-h-0 flex flex-col animate-fade-in-up">
+    <div key={routeKey} className="flex-1 min-h-0 flex flex-col animate-fade-in">
       <Outlet />
     </div>
   )