feat: support Stellar Asset Contracts (SACs)

[Hollujay]

Summary

SACs are currently unsupported. They have no deployed WASM, so the existing
spec-fetching path in contractSpec.ts has nothing to read. SACs are a large
share of the contracts a user might actually want to inspect (every classic
asset has one), so this is the most visible gap in coverage.

Approach

SAC interfaces are known and fixed rather than discovered from WASM. The likely
fix is to detect a SAC and fall back to a built-in interface definition instead
of fetching a spec.

Acceptance Criteria

• Detect that a given contract address is a SAC
• Fall back to the known SAC interface rather than attempting spec fetch
• Functions are listed and callable for a SAC the same way as a custom contract
• The storage diff still works, or the UI clearly explains why it can't for SACs
• Update the README's "What this is (and isn't)" section, which currently
  states SACs are unsupported

Tech Stack

TypeScript, @stellar/stellar-sdk