fix: make message nav tooltip scrollable for long session lists

[usvimal]

Summary

• Add max-height and overflow-y: auto to the message nav tooltip content
• Allows scrolling within the tooltip when there are many messages in a session

Problem

On smaller screens (below 88rem/1408px), the message navigation rail shows in compact mode with tick marks. Hovering reveals a tooltip with the full message list. When there are many messages, the tooltip content overflows but cannot be scrolled, making it impossible to see/select later messages.

Solution

Added to [data-slot="message-nav-tooltip-content"]:

max-height: min(400px, 60vh);
overflow-y: auto;

This limits the tooltip height to 400px or 60% of viewport height (whichever is smaller) and enables vertical scrolling.