### 🔐 Sanitize user-provided transaction inputs

**Overview**  
All transaction inputs should be sanitized before building contract calls.

**Requirements**
- Validate amount values
- Reject negative, zero, NaN, or malformed inputs
- Normalize valid inputs before transaction building

**Acceptance Criteria**
- Unsafe inputs are rejected
- Valid inputs are processed correctly
- Tests cover edge cases

**Labels**: security, transactions