feat: add member management to group treasury

[KingFRANKHOOD]

PR Summary: Add member management to Group Treasury (Closes #41)

Description

Implements member tracking for the group_treasury contract. The contract now manages a whitelist of members who can deposit, with admin-only controls for adding/removing members.

Changes

New contract crate: contracts/contracts/group_treasury/

• storage.rs — DataKey enum (Admin, Members, Balance(Address)) plus MemberAddedEvent and MemberRemovedEvent event structs.
• lib.rs — Eight public functions:
• initialize(admin, members: Vec) — one-time init with initial member list
• add_member(member) — admin only, prevents duplicates
• remove_member(member) — admin only, panics "cannot remove admin" on self-removal, emits MemberRemovedEvent
• deposit(from, amount) — requires membership + from.require_auth(), increments persistent balance
• balance(address), members(), admin() — view functions
• test.rs — 10 unit tests covering:
• Happy path initialize / add / remove / deposit
• Double-init guard
• Duplicate member rejection
• Non-member deposit → "unauthorized: not a member"
• Admin self-removal → "cannot remove admin"
• Removed member: balance preserved, future deposits blocked

Acceptance Criteria Checklist

• 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"

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[codebestia]

Please resolve the conflicts