feat: signPsbt — Bitcoin PSBT signing

[satoshai-dev]

Summary

Add signPsbt wallet method to sign Bitcoin Partially Signed Bitcoin Transactions (PSBTs).

Context

Required for any Bitcoin L1 interactions: sBTC deposits, taproot vaults, ordinals, BRC-20 operations. This is a core method in the Xverse/Sats Connect wallet interface.

Requirements

• Implement signPsbt handler in mock-provider.ts
• Use bitcoinjs-lib (or similar) for PSBT parsing and signing
• Support segwit and taproot inputs
• Bitcoin address derivation from the same mnemonic/key
• Return signed PSBT hex
• Unit tests for PSBT signing
• Test dApp page + E2E spec

Dependencies

• New dependency: bitcoinjs-lib or equivalent
• Bitcoin regtest/testnet4 for E2E testing

Milestone

v0.5