feat(escrow_contract): implement two-step admin transfer mechanism

[Ogstevyn]

Summary

• Added propose_admin so the current admin can nominate a new admin address (stored as pending_admin in instance storage)
• Added accept_admin so the nominated address can claim the admin role; atomically updates admin and clears pending_admin on acceptance
• init now stores the initializer as the first admin
• Added get_admin view function needed to verify admin state
• Emits AdminTransferred { old_admin, new_admin } event on successful transfer
• Added four unit tests covering: happy-path transfer, non-pending address rejection, atomic state update, and event emission

Test plan

• test_init_and_get_status — existing test still passes
• test_propose_and_accept_admin — end-to-end two-step transfer succeeds
• test_accept_admin_rejected_for_non_pending — panics when wrong address calls accept_admin
• test_admin_cleared_after_transfer — old admin no longer set, new admin confirmed
• test_admin_transfer_emits_event — event published on transfer

Closes #14

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