feat(frontend): add taproot PSBT signing utility for browser wallets

[joelklabo]

Summary

Adds a frontend utility module for the Taproot escrow workstream in #230, focused on the two integration pieces discussed in the issue thread:

• keypair generation for Taproot-compatible x-only pubkeys
• browser wallet PSBT signing adapter with provider detection and encoding normalization

What Changed

• Added frontend/src/utils/taproot.ts:
  • generateTaprootKeypair() for deterministic/generated secret -> x-only pubkey
  • isLikelyPsbt() and normalizePsbt() for base64/hex PSBT handling
  • getBrowserPsbtSigner() for browser provider detection (unisat, okxwallet.bitcoin)
  • signPsbtWithBrowserWallet() wrapper that signs via browser wallet and returns normalized base64 PSBT
• Added tests: frontend/src/utils/taproot.test.ts
• Exported helpers from frontend/src/utils/index.ts

Validation

• npm run lint -- src/utils/taproot.ts src/utils/taproot.test.ts src/utils/index.ts
• npm test -- src/utils/taproot.test.ts --runInBand
• npm run build

Notes

• This is intentionally scoped as a utility layer to unblock frontend integration work for Taptrade paths without coupling to backend order-state changes in this PR.

[joelklabo]

Hi @ReneSmeets @cryptosharks - checking in on this PR. It's ready to merge and implements the PSBT signer utilities for the bounty (600k sats). Let me know if any changes are needed!

[joelklabo]

Hi @ReneSmeets @cryptosharks - just checking in again. This PR is ready to merge and implements the PSBT signer utilities for the 600k sats bounty. Is there anything else needed from my side? Thanks!

[joelklabo]

Hi @ReneSmeets @cryptosharks - checking in again. This PR is mergeable and implements the PSBT signer utilities for the 600k sats bounty. Is there anything else needed to move this forward? Thanks!