Add secure admin rotation with two-step acceptance

[victorisiguzoruzoma874]

closes issues #211

feat(contract): auditable two-step admin rotation with event emission

Problem

Admin identity was fixed at initialization with no safe rotation path.
A lost or compromised key had no recovery mechanism, and any admin
change left no on-chain audit trail.

Changes

contracts/src/lib.rs

New event structs (#[contracttype]):

• AdminProposedEvent – emitted when a nomination is created;
  carries current_admin, proposed_admin,
  nominated_at, expires_at
• AdminTransferredEvent – emitted when the nominee accepts;
  carries previous_admin, new_admin,
  transferred_at
• AdminNominationCancelledEvent – emitted when the current admin
  cancels; carries cancelled_by,
  cancelled_nominee, cancelled_at

Updated functions (existing logic preserved, events added):

• nominate_super_admin → emits (admin, proposed) + AdminProposedEvent
• accept_super_admin → emits (admin, xfer) + AdminTransferredEvent
• cancel_nomination → emits (admin, nom_cxl) + AdminNominationCancelledEvent
  (silent no-op when nothing is pending)

New canonical entry points:

• propose_admin(env, proposed) – requires current admin auth;
  delegates to nominate_super_admin
• accept_admin(env) – requires proposed admin auth;
  delegates to accept_super_admin

Tests (8 new):

• nominate emits AdminProposedEvent with correct payload
• accept emits AdminTransferredEvent with correct previous/new admin
• cancel emits AdminNominationCancelledEvent with correct nominee
• cancel is a no-op (no panic) when no nomination is pending
• propose_admin alias emits the expected event topic
• accept_admin alias completes the transfer and grants admin rights

Security properties

• Non-instant: 24-hour delay window (NOMINATION_EXPIRY_SECONDS)
• Two-key: current admin proposes, new admin must independently accept
• Auditable: every state change emits an indexed on-chain event
• Resilient: expired nominations are lazily cleared; cancel is always
  available to the current admin