Title: Show exact dates for sessions in Code tab sidebar, not just Today/Yesterday/Older Body:

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

When grouping sessions by date in the Code tab sidebar, sessions are bucketed into "Today," "Yesterday," and "Older" only. There's no way to see the exact date a session was last worked on without opening it.

### Proposed Solution

Request: For sessions in the "Older" bucket (or as an alternative grouping/sort option), show the actual date (e.g., "Sep 3" or "2026-09-03") instead of lumping everything into "Older." Alternatively, a hover tooltip on each session row showing the exact last-activity timestamp would also solve this.

### Alternative Solutions

I asked Claude to show the dates, and it suggested this action

### Priority

Medium - Would be very helpful

### Feature Category

Interactive mode (TUI)

### Use Case Example

I want work on a recent session, and it helps to locate it if I can see the last date/time each session was accessed

### Additional Context

_No response_

Start by locating the Code tab sidebar's session grouping and date-bucketing entry point. Check how session last-activity timestamps are represented, then implement exact dates for older sessions or a timestamp tooltip and verify the resulting presentation in the interactive TUI.

Written by the indexing model from the issue text.