Support exporting Iron Fish spending key / add 12-word to 24-word conversion

[jiangwei221]

Description

I have an Iron Fish wallet created in Fox Wallet iOS with a 12-word seed phrase. I want to import this wallet into the official Iron Fish GUI wallet, but the official wallet requires a 24-word mnemonic phrase.

Problem

1. Fox Wallet iOS uses 12-word BIP39 mnemonics (128-bit entropy) for Iron Fish
2. The official Iron Fish wallet only accepts 24-word mnemonics (256-bit entropy)
3. There is no way to export the raw spending key from Fox Wallet to import directly

Request
Either:

1. Add a "Export Spending Key" option in Fox Wallet settings that displays the raw 64-character hex spending key, so users can import via:
   ironfish wallet:import --spendingkey
2. Open source the code that derives the Iron Fish spending key from the 12-word mnemonic, so the community can verify the key derivation is correct

Context

• Official Iron Fish import accepts raw spending keys (64 char hex) as well as mnemonics
• Users should have sovereignty over their keys - being forced to use a specific wallet to access funds is a centralization concern