Add two-step admin transfers to invoice and treasury

[Niceguy247]

Summary

• Add two-step admin handoff to the invoice and treasury contracts: the current admin nominates a successor, and only that nominated address can authorize acceptance.
• Block admin transfers while paused, clear pending nominations after acceptance, and ensure the previous admin loses privileges after handoff.
• Emit transfer lifecycle events and update ABI snapshots and mainnet deployment documentation.
• Add tests for successful handoff, authorization failures, paused contracts, event emission, and pending-admin cleanup.

Validation

• git diff --check passed.
• Rust contract tests were not run because cargo is unavailable in the environment.

No issue IDs were provided for linking or auto-closing.
Fixes #662
Fixes #661
Fixes #660
Fixes #663

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