feat(invoice-token): implement account freeze functionality (#98, #100, #105, #107)

[chizzy192]

Context & Goal

Issues #98, #100, #105, #107 collectively request account freeze/unfreeze in InvoiceToken. A frozen account must be rejected on all transfer, burn, and transfer_from operations via StorageKey::FrozenAccount(Address).

Work Required

• storage.rs: add FrozenAccount(Address) and is_frozen/set_frozen helpers.
• lib.rs: add set_frozen(address, frozen) (admin-only). Add frozen check to transfer, transfer_from, burn, burn_from.
• errors.rs: add AccountFrozen with unique discriminant.
• Tests: freeze blocks transfer/burn, unfreeze restores, non-admin freeze rejection, frozen as recipient (succeeds).

Acceptance Criteria

• Frozen accounts rejected on all transfer/burn paths.
• Events emitted correctly.
• cargo test -p invoice-token passes with no regressions. PR targets dev.

Target Branch: dev
Estimated Effort: 5–12 hours | Delivery: 3–5 days

[chizzy192]

Resolved and closed as completed. Implemented and merged in PR #319 and PR #326 (Invoice token account freeze controls and zero-address rejection).