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/EntitySelect.svelte

@@ -304,6 +304,7 @@
 	/* List */
 	.ep-list {
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 		padding: 0.35rem;
 		position: relative;

frontend/src/lib/components/IconGridSelect.svelte

@@ -195,6 +195,7 @@
 		padding: 0.5rem;
 		max-height: 320px;
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 	}
 	:global([data-theme="light"]) .icon-grid-popup { --igs-solid-bg: #fafafe; }

frontend/src/lib/components/IconPicker.svelte

@@ -188,6 +188,7 @@
 		max-height: 14rem;
 		overflow-y: auto;
 		overflow-x: hidden;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 		position: relative;
 		z-index: 1;

frontend/src/lib/components/Modal.svelte

@@ -192,6 +192,7 @@
 		z-index: 1;
 		padding: 0 1.5rem 1.5rem;
 		overflow-y: auto;
+		overscroll-behavior: contain;
 	}
 
 	.modal-close {

frontend/src/lib/components/MultiEntitySelect.svelte

@@ -307,6 +307,7 @@
 
 	.mes-list {
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 		padding: 0.25rem 0;
 	}

frontend/src/lib/components/SearchPalette.svelte

@@ -342,6 +342,7 @@
 	.sp-results {
 		max-height: 52vh;
 		overflow-y: auto;
+		overscroll-behavior: contain;
 		scrollbar-width: thin;
 		padding: 0.35rem;
 		position: relative;

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. */