refactor(contracts): rename confusing NotAnAdmin error to AccountNotAdmin ♻️

[Klazomenai]

Problem

Two custom errors have near-identical names:

• NotAdmin (line 17) — "caller is not an admin" (used by onlyAdmin modifier)
• NotAnAdmin (line 20) — "target account is not an admin" (used by removeAdmin)

These names are confusing when reading ABI error selectors or debugging reverts. Integrators can't distinguish them at a glance.

Found during security audit (2026-03-23).

Acceptance Criteria

• NotAnAdmin renamed to AccountNotAdmin (or TargetNotAdmin)
• All references updated (contract + tests)
• forge test passes

Implementation

// Before
error NotAnAdmin();

// After
error AccountNotAdmin();

Parent: #16