Rework session list: highlight active work, de-emphasize idle projects, foldable groups

[SGudbrandsson]

Problem

The session list currently shows every project with its sessions nested underneath. Projects without active sessions just take up space and make it harder to see what's actually being worked on.

Proposal

Rethink the visual layout of the sidebar / session list to:

• Highlight in-progress work (active sessions, recent activity) at the top or visually emphasized.
• De-emphasize idle projects (collapsed, muted, or hidden until expanded).
• Add foldable project groups — users can group related projects together and open/close the group as a unit.
• Empty projects shouldn't crowd the view.

Open questions

• Should "activity" be based on last message, running process, or something else?
• How are groups defined — user-created, automatic by folder, tags?
• Do we keep a "show all projects" affordance for discoverability?

Acceptance criteria

• Active sessions visually distinct from idle ones.
• Projects with no sessions are collapsed or hidden by default.
• User can create/collapse/expand project groups.