Group hero avatar: let user pick which 1–2 members appear

[tashda]

Summary

The Group Card hero shows an avatar built from the first two members of the group. There was no way to influence the choice — for groups with many members the icon would be whatever happened to be first in the membership list, often not the most recognisable two.

Fix (commit on dev)

Tap the hero avatar on the Group Card to open a member-picker sheet:

• Lists every group member with image, name, model.
• Tap to toggle selection. Max two selected; tapping a third replaces the earliest pick (numeric badge shows the order, "1" and "2", so the user can see which slot they're filling).
• "Reset" returns to the default (first two).
• "Done" saves and dismisses; selection persists in UserDefaults under group.avatar.<groupID>.

GroupIconView keeps its original API. The Group Card resolves the avatar via a new GroupAvatarStore that filters out IEEEs no longer in the group and falls back to the first-two default when no selection exists. The smaller compact-row icon and the device-list group row remain pure list rows — only the prominent hero is interactive.

Acceptance criteria

• Tapping the hero icon on a Group Card opens the picker.
• Selecting two members and tapping Done changes the hero avatar.
• Removing a selected member from the group quietly falls back to defaults instead of showing a missing icon.
• Restarting the app preserves the choice.