[FEATURE] Allow "Last activity" time filter when Group by is set to Custom groups

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The Sessions panel offers a "Last activity" filter (1d / 3d / 7d / 30d / All) but only when Group by is set to State. The moment I switch Group by to Custom groups, the "Last activity" row disappears from the filter menu entirely, and there is no way to bring it back. Changing Sort by to Recency does not restore it either, since the sort control and the activity filter are separate.
I use custom groups to organize my sessions by client project (SEO, UNICEF Balochistan, EU, SalesAI, Banam Invoices, and so on) so I can navigate between the things I'm actively working on. This is exactly the workflow custom groups are meant to support. But because the activity filter is unavailable in this mode, every group shows every session ever created in it, all the way back to inception. Long-running project groups become very hard to navigate, and the organizational benefit of custom groups is undercut by the clutter.
Right now I'm forced to choose between two things that should not be mutually exclusive:

1. Custom groups (my project organization), with no way to hide stale sessions, or
2. The activity filter, but only in State mode, which throws away my grouping.

<img width="204" height="124" alt="Image" src="https://github.com/user-attachments/assets/46b86600-8eaf-4ce1-bc21-0dd71aa75ab8" />

<img width="202" height="140" alt="Image" src="https://github.com/user-attachments/assets/8215fa97-b935-49ac-8b36-cfcd8d19be54" />

Proposed Solution

Make the "Last activity" filter available in Custom groups mode (and ideally in all Group by modes). The filter should apply within each group, so I can, for example, see only sessions active in the last 7 or 30 days inside each of my project groups, while keeping the groups intact.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

Collapsing groups and sorting by Recency helps a little, but it only reorders sessions, it doesn't hide old ones. The activity window is the piece that actually keeps a busy group navigable.