Add offline build variants for write methods and XDR submission helper

[yuliuyi717-ux]

Related to #42

Implements offline transaction building support for write methods and adds signed XDR submission.

What is included:

1. Offline build support
   • Added a generic buildOffline(operation, sourcePublicKey, sequenceNumber) method
   • Added *BuildOffline(...) variants for all write methods:
     • createGroupBuildOffline
     • joinGroupBuildOffline
     • leaveGroupBuildOffline
     • startGroupBuildOffline
     • contributeBuildOffline
     • distributePayoutBuildOffline
     • pauseGroupBuildOffline
     • resumeGroupBuildOffline
     • raiseDisputeBuildOffline
2. Sequence number input handling
   • Added AccountSequence type (string | number | bigint)
   • Added robust sequence normalization/validation before building offline transactions
3. Deferred submission support
   • Added submitSignedTransaction(signedXdr) to submit pre-signed envelopes later
   • Returns normalized { hash, status }
4. Type exports
   • Added SubmittedTransaction type
   • Exported new types from src/index.ts

Acceptance criteria mapping:

• buildOffline variants for all write methods: ✅
• accept pre-fetched account sequence number: ✅
• return unsigned XDR for later signing/submission: ✅
• add submitSignedTransaction(xdr): ✅

Validation:

• npm run build (TypeScript compile) passes.