Web SDK: DevEx improvements for contracts

## Summary

Improve the developer experience for working with contracts in the web client.

### Tasks

- [ ] Simplify the contracts API
- [ ] Add out-of-the-box support for MASP artifacts
- [ ] Improve error handling for contract interactions
- [ ] Add performance metrics

The issue is about the web SDK's contracts API. Look at the existing contract interaction code in the SDK to understand the current API surface. Investigate how MASP artifacts are currently handled, if at all. Review error handling patterns in contract calls and identify what metrics could be collected. The scope is broad and requires design decisions.

Written by the indexing model from the issue text.