feat: Add peek animation and hide scrollbars in DM lists

- Add `.no-scrollbar` utility to hide scrollbars while keeping scrollability.
- Apply `.no-scrollbar` to ConversationList and MessageThread containers.
- Implement "peek" animation:
    - ConversationList: scrolls down 40px then back up on load.
    - MessageThread: scrolls up 40px then back down (to bottom) on load.
- Animation only triggers if content overflows.

css/tailwind.source.css

@@ -5984,3 +5984,11 @@
     }
   }
 }
+
+.no-scrollbar {
+  scrollbar-width: none;
+}
+
+.no-scrollbar::-webkit-scrollbar {
+  display: none;
+}