[FEATURE] Code tab sidebar: per-group view controls (group by date, sort) on every custom group

Context

I organise about 320 Code sessions into 35 custom sidebar groups, one per client project or internal track. The groups work well for filing, but large groups are hard to scan.

Request: view options on every custom group

Today the view options icon (two sliders, with Status, Environment, Group by, Sort by) only appears on the header of the first group in the sidebar, and it changes the whole sidebar. Large groups (30+ sessions) stay one long flat list.

I would like the same icon on every custom group's header, with options that apply only to that group:

• Group by: None · Date (Today / This week / Older) · State
• Sort by: Last activity · Name · Created

Observation that suggests this is close

The per-group setting already seems to exist in the data model. How to reproduce:

1. In a Code session, ask Claude to call the built-in sidebar tool set_view with a custom group's section_id and group_by: "date".
2. list_sections then reports group_by: "date" for that group.
3. The sidebar still shows that group as a flat list, with no date headings.

So the setting is stored, but the sidebar does not render it, and there is no UI to set it per group.

Why it matters

Without a per-group view, the only way to tidy a long group is to collapse it, and collapsed groups get forgotten along with the open work inside them.