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/state/scheduledMessages.ts

@@ -1,4 +1,5 @@
 import { atom } from 'jotai';
+import { atomWithStorage, createJSONStorage } from 'jotai/utils';
 import { IContent } from 'matrix-js-sdk';
 
 export type ScheduledMessage = {
@@ -8,9 +9,34 @@ export type ScheduledMessage = {
   sendAt: number; // Unix timestamp ms
 };
 
+const STORAGE_KEY = 'cinny_scheduled_messages_v1';
+
+// Internal atom persists as a plain Record (JSON-serializable).
+const internalAtom = atomWithStorage<Record<string, ScheduledMessage[]>>(
+  STORAGE_KEY,
+  {},
+  createJSONStorage(() => localStorage),
+);
+
 /**
  * Global atom: Map<roomId, ScheduledMessage[]>
  * Stores all locally-tracked scheduled messages across rooms.
  * MSC4140 has no list endpoint, so we track them ourselves.
+ * Backed by localStorage so scheduled messages survive page refreshes.
  */
-export const scheduledMessagesAtom = atom<Map<string, ScheduledMessage[]>>(new Map());
+export const scheduledMessagesAtom = atom(
+  (get): Map<string, ScheduledMessage[]> => new Map(Object.entries(get(internalAtom))),
+  (
+    _get,
+    set,
+    updater:
+      | Map<string, ScheduledMessage[]>
+      | ((prev: Map<string, ScheduledMessage[]>) => Map<string, ScheduledMessage[]>),
+  ) => {
+    set(internalAtom, (prevObj) => {
+      const prevMap = new Map(Object.entries(prevObj));
+      const nextMap = typeof updater === 'function' ? updater(prevMap) : updater;
+      return Object.fromEntries(nextMap.entries());
+    });
+  },
+);