Improve timeline scroll, media layout, avatars, and emoji coverage.

Remember room scroll position across switches, reserve image/video heights from Matrix dimensions, cache authenticated media blobs for avatars, restore jumbo emoji-only messages, and extend emoji font unicode-range for Unicode 15 glyphs.

src/app/state/roomScrollCache.ts

@@ -0,0 +1,32 @@
+export type RoomScrollRange = {
+  start: number;
+  end: number;
+};
+
+export type RoomScrollState = {
+  top: number;
+  atBottom: boolean;
+  range: RoomScrollRange;
+};
+
+const cache = new Map<string, RoomScrollState>();
+
+const MAX_ENTRIES = 20;
+
+export const getRoomScrollState = (roomId: string): RoomScrollState | undefined =>
+  cache.get(roomId);
+
+export const saveRoomScrollState = (roomId: string, state: RoomScrollState): void => {
+  if (cache.has(roomId)) cache.delete(roomId);
+  cache.set(roomId, state);
+
+  while (cache.size > MAX_ENTRIES) {
+    const oldest = cache.keys().next().value as string | undefined;
+    if (!oldest) break;
+    cache.delete(oldest);
+  }
+};
+
+export const clearRoomScrollState = (roomId: string): void => {
+  cache.delete(roomId);
+};