[CONTRACT] Add member management to Group Treasury

[codebestia]

Add member tracking to group_treasury. initialize takes an initial Vec<Address> of members. Add add_member(member) and remove_member(member) (admin only). Only members can call deposit. Emit MemberAddedEvent and MemberRemovedEvent.

Acceptance criteria:

• Non-member deposit panics with auth error
• Removing a member prevents future deposits but does not affect balance
• Admin removal of self panics with "cannot remove admin"

[KingFRANKHOOD]

@KingFRANKHOOD has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello, I’m a full stack developer with hands-on experience building with variety of tools including Nextjs, nodejs, nestjs, React, Typescript and more

I focus on writing clear, maintainable code and making sure features work as expected end to end, not just in isolation. I communicate progress regularly and keep things transparent as I work. I’m confident I can complete this task within an ETA of 12 HOURS

ℹ️ Repo Maintainers: To accept this application, review their application or assign @KingFRANKHOOD to this issue.

[drips-wave]

Congratulations, @KingFRANKHOOD! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @KingFRANKHOOD: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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