Issue 18 — mint/burn silently succeed on non-positive amount

[Junman140]

Labels: bug, priority:medium
Body:

Problem
Both functions return Ok(()) when amount <= 0 instead of returning an error. A relayer bug producing a zero-amount mint call will pass silently instead of surfacing as a defect.

Proposed fix
Add Error::InvalidAmount and return it for amount <= 0, forcing callers (the relayer) to handle it explicitly.

Acceptance criteria

• Zero/negative amount returns an explicit error
• Test covers both cases

---