Feat/81 offline stellar signing

[Hollujay]

Closes #81

PR Summary

Offline Signing Workflow for Stellar

Adds three functions for cold-signing Stellar stealth payments:

• prepareOfflineStellarTransaction — builds an unsigned transaction envelope offline, returns a serializable OfflineStellarEnvelope with XDR, network passphrase, and hash
• signOfflineStellarTransaction — signs an envelope with either a Stellar secret key (S...) or a stealth private scalar (bigint + stealthPubKey for the signature hint)
• submitOfflineStellarTransaction — POSTs the signed XDR to Horizon via getDeployment

New Files

File	Description
src/chains/stellar/offline-sign.ts	3 exported functions + OfflineSignParams / OfflineStellarEnvelope types
test/chains/stellar/offline-sign.test.ts	11 unit tests (no mocks, real Keypair)
docs/offline-signing.md	Cold-signing workflow guide with stealth address compat and security best practices

Changes to Existing Files

• src/chains/stellar/index.ts — re-exports the new module
• src/chains/stellar/stealth.ts — removes duplicate computeAnnouncementViewTag declaration (pre-existing bug fix)
• .prettierignore — adds announcements.ts (pre-existing syntax error)

Test Coverage

• Valid envelope fields (transactionXdr, networkPassphrase, hash)
• Input validation (empty source, ops, sequence, passphrase)
• Regular secret key signing
• Stealth scalar + stealthPubKey signing (full deriveStealthKeys → generateStealthAddress → deriveStealthPrivateScalar → signOfflineStellarTransaction flow)
• Missing stealthPubKey error
• Invalid envelope error
• submitOfflineStellarTransaction existence as separate async function
• End-to-end: prepare → sign → parse → verify ops and signatures.

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[truthixify]

Almost there. After #99 just landed, conflicts in .prettierignore and src/chains/stellar/index.ts.

git fetch origin
git rebase origin/develop
# .prettierignore: keep both sets of entries
# index.ts: keep develop's sdk-react exports, then add your offline-sign export
git add -u
git rebase --continue
git push --force-with-lease

[Hollujay]

@truthixify Rebased on develop and force pushed. Conflicts resolved

[truthixify]

Clean merge after rebase. 211-line offline-sign + 231-line tests + 244-line docs. Hardware-wallet / cold-storage workflow is exactly what was asked. Thanks @Hollujay.