Build Mobile Wallet SDK (iOS/Android)

[Steake]

Part of Epic: #75 - RC2: Wallet & Security Infrastructure

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Goal: Deliver cross-platform wallet SDK with secure key management, backup/restore, and FFI bindings.

Requirements:

• iOS/Android core SDK
• Keychain/Keystore integration
• Biometrics, backup/restore flows

Acceptance Criteria:

• App integrates and signs transactions
• All key management flows tested

See [RELEASE_REQUIREMENTS.md](https://github.com/Steake/BitCell/blob/master/docs/RELEASE_REQUIREMENTS. md)

[Steake]

[autopilot] @copilot please start work on this issue and open a PR that references #58 in the PR description.