Support funding Ledger hardware wallet addresses via stellar keys fund --ledger #2555

Summary

It is currently not possible to fund a Ledger hardware wallet address using the CLI. Running stellar keys fund requires an identity that resolves to a local keypair; there is no --ledger flag, so Ledger users must derive their address out-of-band and fund it via other means.

Motivation

Hardware wallets are a primary key-management option in the CLI (stellar keys add --ledger), and funding a freshly-derived address on testnet/futurenet is a routine first step in any development workflow. Without this capability, Ledger users face an inconsistent experience — every other key type can be funded directly.

Proposed solution

Add a --ledger flag to stellar keys fund that:

1. Connects to the attached Ledger device and derives the public key for account index 0 (i.e. m/44'/148'/0') by default.
2. Calls Friendbot (or the configured funder) with that address, identical to how funding already works for named identities.
3. Respects --hd-path <N> when provided, where N is the integer account index used to derive the address at m/44'/148'/N'.

Desired usage:

Fund the default Ledger account (index 0):
  stellar keys fund --ledger

Fund a specific account index:
  stellar keys fund --ledger --hd-path 1

Acceptance criteria

• stellar keys fund --ledger connects to a Ledger device, derives the public key at index 0, and funds the corresponding account via Friendbot.
• stellar keys fund --ledger --hd-path N funds the account derived at the given integer index.
• A clear error is shown when no Ledger device is connected or the Stellar app is not open on the device.
• A clear error is shown when Friendbot is unavailable (consistent with the existing behaviour for --fund on keys generate).
• The funded address is printed to stdout so the user can confirm which account was topped up.