sbtc: update @scure/btc-signer dependency to v2.x

[pbtc21]

Summary

The sbtc npm package (v0.3.2) uses @scure/btc-signer@^1.4.0, which is now outdated. The latest version is 2.0.1. This causes dependency conflicts when using sbtc alongside other packages that use v2.x.

Current Behavior

// sbtc package.json
"dependencies": {
  "@scure/btc-signer": "^1.4.0"  // resolves to 1.8.1
}

When installed in a project using @scure/btc-signer@2.x, npm creates nested dependencies:

node_modules/
  @scure/btc-signer/  (2.0.1 - project's version)
  sbtc/
    node_modules/
      @scure/btc-signer/  (1.8.1 - sbtc's nested version)

Bugs Found

1. fetchSignersAddress() returns regtest format - The function hardcodes REGTEST network:

   // In api.js
   return btc.p2tr(pub, undefined, constants_1.REGTEST).address;
   // Returns: bcrt1p... instead of bc1p...

2. fetchSbtcBalance() sometimes fails - JSON parsing errors, possibly due to API changes

Expected Behavior

• Update @scure/btc-signer to ^2.0.0
• Fix fetchSignersAddress() to use the correct network based on client type (Mainnet/Testnet)
• Ensure API compatibility with current Hiro endpoints

Environment

• sbtc version: 0.3.2
• @scure/btc-signer (project): 2.0.1
• @scure/btc-signer (sbtc nested): 1.8.1

Additional Context

The sbtc package was last published about a year ago. With sBTC now live on mainnet and withdrawals enabled, having an up-to-date SDK would be valuable for developers building sBTC integrations.

Related: We're building agent tooling at aibtc-mcp-server that uses both the sbtc package and @scure/btc-signer v2 for inscriptions/ordinals.