Implement offline transaction building

[big14way]

Allow building transactions without network access, then submit later.

Acceptance Criteria:

• Add buildOffline() variants for all write methods
• Accept pre-fetched account sequence number
• Return unsigned XDR that can be signed and submitted later
• Add submitSignedTransaction(xdr) method

Files: src/client.ts, src/types.ts

[lustsazeus-lab]

This is a good candidate for a 2-phase API:

1. buildOffline(...) returns unsigned XDR + metadata (network passphrase, expected seq),
2. submitSignedTransaction(xdr) validates that metadata before broadcast.

If you also include a stale-sequence error type in the result contract, wallets can recover cleanly after long offline windows.

[yuliuyi717-ux]

I'm taking this issue as the next lane and will open a PR shortly.

Planned scope:

1. add buildOffline variants for all write methods
2. accept caller-provided account sequence number
3. return unsigned XDR for later signing
4. add submitSignedTransaction(xdr) helper for delayed submission

[yuliuyi717-ux]

Opened PR: #75

Delivered in this PR:

• buildOffline support + *BuildOffline variants for all write methods
• caller-provided sequence number support
• unsigned XDR output for delayed signing/submission
• submitSignedTransaction(xdr) helper

Validation: npm run build passes.