#42901 regression: secondary sidebar item adjustments after the `Item` migration

Proposed changes (including videos or screenshots)

ARCH-2499
Fixes layout regressions in the secondary sidebar feature preview introduced by #42819:

• Rows did not reach the edges of the list. The row wrapper and the filter tablists added paddingInline={4}, so the hover and selected surfaces stopped short of the edges and read as pills. The padding is removed.
• Filters were misaligned with the rooms and group headers. The filters rendered a bare Item without an inset, while rooms and group headers use the md inset. They now render SidebarItem, so all rows share the same inset. The filter dividers follow that inset (marginInline={12}).
• The sticky group header was transparent. Before the migration the collapse group bar carried the sidebar surface; the Item group header does not, so rooms scrolling under the pinned header showed through it. The virtualized group wrapper now takes surface-sidebar.

All changes live in the application, under the feature preview's RoomList; Fuselage and other SidebarItem consumers are untouched.

Issue(s)

Regression from #42819 (unreleased), so no changeset.

Steps to test or reproduce

1. Enable the secondary sidebar feature preview.
2. Hover and select rooms and filters (All, Favorites, Discussions, omnichannel filters): the surface fills the full row width.
3. Check that filter icons, group headers and room avatars line up.
4. Scroll the room list: the pinned group header stays opaque over the rooms passing beneath it.

Further comments

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