refactor: remove dependency on LDK

[ovitrif]

Integrates:

• feat: expose derive_node_secret_from_mnemonic ldk-node#26

This PR removes the dependency on LDK from the migration code by using the new deriveNodeSecretFromMnemonic function exposed by ldk-node instead.

Description

• Removes the bundled LDK-release.aar library (~18MB) which was only used for key derivation during RN migration
• Replaces complex manual BIP39/BIP32 seed derivation with the simpler deriveNodeSecretFromMnemonic function from ldk-node
• Removes reflection-based access to get_node_secret_key which was fragile
• Consolidates duplicate toHex implementations by using the shared extension function
• Updates ldk-node-android to v0.7.0-rc.3

Preview

N/A - No UI changes

QA Notes

• Tested with regtest RN app and regtest debug build of Bitkit Android.
• Tested with prod RN app and mainnet release build of Bitkit Android.

Tests:

1. Migration from RN

1. Install RN Bitkit and setup with a funded wallet
2. In build.gradle.kts:65 clear the string for applicationIdSuffix
   It should be: applicationIdSuffix = ""
3. Install this build over RN app
4. Verify migration completes successfully

[jvsena42]

LGTM, I'll test it when the conflicts are solved