fix(redesign): contain modal scroll chaining and smooth Telegram chat refresh

- Add overscroll-behavior: contain to all in-modal/popup scroll
  containers (Modal body, EntitySelect, MultiEntitySelect, IconPicker,
  IconGridSelect, SearchPalette, TimezoneSelector) so reaching the
  inner scroll boundary no longer scrolls the page underneath.
- Telegram bot Discover Chats no longer collapses the existing chat
  list into a "Loading…" placeholder. Split chatsLoading (initial)
  from chatsRefreshing (Discover); rows are keyed by chat.id with
  flip+fade animations; the list dims with a sweeping shimmer bar
  while the Discover button shows a spinning icon and "Discovering
  chats…" label. Honors prefers-reduced-motion.

frontend/src/lib/i18n/ru.json

@@ -475,6 +475,7 @@
 		"noCommandsForProvider": "Этот тип провайдера не поддерживает команды бота.",
 		"syncCommands": "Синхр. команды",
 		"discoverChats": "Обнаружить чаты из Telegram",
+		"discoveringChats": "Поиск чатов…",
 		"clickToCopy": "Нажмите, чтобы скопировать ID чата",
 		"chatsDiscovered": "Чаты обнаружены",
 		"chatDeleted": "Чат удалён",