Empty group should offer Add Members directly from the placeholder

[tashda]

Summary

A newly-created group with no members showed only a static "No Members — Add devices to this group" placeholder. To actually add a device the user had to go up to the toolbar + menu — extra friction for the most-likely next step.

Fix (commit on dev)

The empty-state ContentUnavailableView is followed by a regular Form-row Button labelled "Add Members" that opens the existing AddGroupMembersSheet. The button takes the iOS-native form-row treatment (tinted icon + system row chrome) — that's the consistent pattern with the rest of the device list / settings actions, and it avoids the centered-floating button issue the action-builder version produced. The toolbar + entry stays available for groups that already have members.

Acceptance criteria

• Open a group with zero members. The "Add Members" row appears beneath the placeholder.
• Tapping it opens the same picker the toolbar uses.
• Once members exist, the empty-state placeholder and the inline button disappear.