fix(chat): keep sidebar chat selection in sync with the active chat (#26977)

When navigating from a chat to a non-chat route (e.g. the admin panel),
the previously-viewed chat stayed selected in the sidebar and
deleting/archiving it wrongly redirected back to the new-chat page.
Cloning a chat also left the source chat highlighted alongside the new
clone, so two chats appeared selected at once.

Two independent sources kept the stale selection:

- The chatId store was never cleared when the Chat component unmounted,
  so $chatId still pointed at the last-viewed chat (this drove the
  delete/archive redirect). Clear chatId/chatTitle in Chat's onDestroy.
- The sidebar's optimistic selectedChatId highlight, set on click, was
  only cleared on window blur (hence it appeared to fix itself after a
  tab switch) and never followed programmatic navigation. Bind it to the
  chatId store so it tracks the active chat for leave, delete and clone.

src/lib/components/chat/Chat.svelte

@@ -1362,6 +1362,13 @@
 				pageSubscribe();
 				showControlsSubscribe();
 				selectedFolderSubscribe();
+
+				// Clear the selected chat when leaving the chat surface (e.g. navigating
+				// to the admin panel), otherwise the previously-viewed chat stays selected
+				// in the sidebar and deleting/archiving it wrongly navigates away.
+				chatId.set('');
+				chatTitle.set('');
+
 				window.removeEventListener('message', onMessageHandler);
 				$socket?.off('events', chatEventHandler);
 				$socket?.off('connect', handleSocketConnect);