Share token refuses renounce_admin

[hpmaxi]

Priority

Medium — should be addressed soon

User story / Problem statement

Currently, the share token keeps OpenZeppelin's default renounce_admin, so its admin can remove itself. The vault and the oracle both refuse this with AdminRequired. On the token, pause, unpause and role management need the admin. A token paused when its admin renounces stays paused for good, and no new vault can ever be granted the manager role.

Expected outcome

The share token refuses renounce_admin the same way the vault and the oracle do. The admin can still hand over through transfer_admin_role and accept_admin_transfer.

Acceptance criteria

• renounce_admin on the share token always fails.
• A test shows the refusal, and that the admin is unchanged afterwards.
• Admin handover in two steps still works on the token.
• The renounce_admin row in docs/AUTHORIZATION_MATRIX.md (§5.2) names the refusal and its test.

Technical notes

The token has no error enum today, so the refusal needs one (for example ShareTokenError::AdminRequired), unless an existing error fits. The vault (contracts/async-vault/src/lib.rs) and the oracle (contracts/nav-oracle/src/lib.rs) show the override.

Additional context

Raised in review of #155.