fix: avatar decoration centering, animation flickering, scheduled message persistence

- UserHero: move AvatarDecoration inside AvatarPresence so the decoration
  inline-flex container sizes to the avatar only, not the presence badge
- SidebarNav: add will-change: background-position, background-size on
  document.body for animated backgrounds, promoting them to a compositor
  layer so overlaid text/UI doesn't repaint on every animation frame
- scheduledMessages: back the atom with atomWithStorage so the scheduled
  message tray survives page refreshes via localStorage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/SidebarNav.tsx

@@ -48,6 +48,7 @@ export function SidebarNav() {
       style.removeProperty('background-size');
       style.removeProperty('background-position');
       style.removeProperty('animation');
+      style.removeProperty('will-change');
       return;
     }
 
@@ -58,6 +59,13 @@ export function SidebarNav() {
     style.backgroundSize = (bgStyle.backgroundSize as string | undefined) ?? '';
     style.backgroundPosition = (bgStyle.backgroundPosition as string | undefined) ?? '';
     style.animation = (bgStyle.animation as string | undefined) ?? '';
+    // Promote animated backgrounds to their own compositor layer so the browser
+    // doesn't repaint the overlaid text/UI content on every animation frame.
+    if (bgStyle.animation) {
+      style.willChange = 'background-position, background-size';
+    } else {
+      style.removeProperty('will-change');
+    }
 
     return () => {
       style.removeProperty('background-image');
@@ -65,6 +73,7 @@ export function SidebarNav() {
       style.removeProperty('background-size');
       style.removeProperty('background-position');
       style.removeProperty('animation');
+      style.removeProperty('will-change');
     };
   }, [glassmorphismSidebar, chatBackground, lotusTerminal, isDark, pauseAnimations]);