Add "Last Active" Date to Sidebar Tooltips

Users can sometimes lose track of their chats. While the sidebar logically groups chats by recent activity (e.g., "Last 7 Days", "Last 30 Days", "Older"), it doesn't give the precise date. Adding visible dates to the sidebar list would ruin the clean, minimalist design of the interface, but users still need context for when they last worked on a thread.

Proposed Solution:
Update the existing hover tooltip on sidebar chat titles (which currently just repeats the title). When a user hovers over a chat, it should display the specific date the user last interacted with it.

• Example Tooltip text: "History Lessons... (Last Active: May 12, 2026)"

This keeps the UI clean while giving users exact clarity.