docs: add Stellar offline transaction signing guide

[baedboibidex-cmyk]

PR Message: Stellar offline transaction signing guide (#40)

Summary

Adds guides/stellar-offline-signing.mdx — a complete guide for building Stellar transactions online, signing them on an air-gapped machine, and submitting them without the private key ever touching an internet-connected device.

What changed

• ** New page:** guides/stellar-offline-signing.mdx covering:
  • Threat model: what offline signing mitigates and what it does not
  • Three-step workflow: build offline → sign offline → submit online (with full TypeScript examples)
  • Transaction inspection before signing (inspectTransaction pattern)
  • QR code transfer patterns (generate, scan, return signed XDR)
  • USB drive fallback for camera-restricted environments
  • Hardware wallet integration (Ledger) with honest current limitations
  • Stealth withdrawal specific guidance: viewing key vs. spending key separation, re-deriving the stealth scalar fully offline
• ** docs.json** — page registered under Guides → Operations
• ** sdk/chains/stellar.mdx** — See Also cross-link added
• ** guides/privacy-best-practices.mdx** — See Also cross-link added

Acceptance criteria

• Page committed
• End-to-end workflow demonstrated (build → QR/USB transfer → inspect → sign → transfer back → submit)
• Hardware wallet section honest about current limitations — explicit <Warning> block states that Ledger/Trezor cannot yet derive Wraith stealth scalars; documents what hardware wallets CAN do today (sign with regular Stellar keypair as a multisig signer)
• Linked from SDK Stellar page
• Linked from Security overview (privacy-best-practices)

Design decisions

• SDK Primitives vs. Helpers: prepareOfflineStellarTransaction not yet in SDK — the issue notes "SDK is gaining" this helper. Documented the pattern using existing Stellar SDK primitives (TransactionBuilder, Transaction, xdr.DecoratedSignature) and added a <Note> block indicating the helper is coming as a drop-in replacement.
• QR Code Tooling: No single standard exists; used qrcode (most common npm package) for generation and zbarcam for scanning, with a USB fallback section for camera-restricted environments. Avoided prescribing a single mandatory tool.
• Viewing Key Online, Spending Key Offline: The guide explicitly shows how to run the announcement scanner with only the viewing key on the online machine, keeping the spending key and derived scalar fully air-gapped.

Follow-ups

• Update SDK helper section once prepareOfflineStellarTransaction ships
• Add hardware wallet stealth scalar derivation section once firmware support lands
• Consider a companion guide for multi-device QR workflows (phone as QR bridge between two desktops)

Closes #40

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[truthixify]

Clean merge. 585-line offline signing guide with airgap workflow + verification path is solid. Thanks @baedboibidex-cmyk.