gui/circles: fix null-deref crash, wrong circle ID in accept/reject, deferred status update, and thread safety in CreateCircleDialog

[jolavillette]

• Fix null-deref crash in [MembershipListCustomPopupMenu] — added early return when no item is selected
• Fix [acceptInvite]/[rejectInvite] — used circle group ID instead of member key ID
• Fix "Pending" display bug — deferred [updateMembership()] to a single call after all members are loaded in [fillIdentitiesList]
• Init am_I_* variables in constructor to prevent undefined behavior
• Reset am_I_circle_admin at the start of [updateMembership()] to avoid stale state
• Thread safety — replaced RsThread::async + usleep + postToObject with QTimer::singleShot (auto-cancelled on dialog destruction)