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/components/TimezoneSelector.svelte

@@ -530,6 +530,7 @@
 
 	.tz-list {
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		/* No top padding — the sticky group head is at top:0 of the
 		   scroll container, so any padding-top would let scrolling
 		   items leak into the gap above the sticky header. */