Ensure chat key fetches bypass cache

Added `{ cache: 'no-store' }` to fetch requests for chat keys in both the chat page and useSodiumChat hook to prevent stale key data. Also updated the chat key query to order by `lastSeenAt` descending, ensuring the most recent bundle is retrieved.

src/lib/hooks/useSodiumChat.ts

@@ -64,7 +64,7 @@ export function useSodiumChat() {
           if (!user.did) {
             throw new Error('User DID not available');
           }
-          const checkResponse = await fetch(`/api/chat/keys?did=${encodeURIComponent(user.did)}`);
+          const checkResponse = await fetch(`/api/chat/keys?did=${encodeURIComponent(user.did)}`, { cache: 'no-store' });
 
           let shouldPublish = false;