Issue 12: Support separate Friendbot / custom faucet URLs per network

[Nanle-code]

Labels: enhancement, network

Description:
Friendbot URL is hardcoded in src/utils/horizon.rs; custom testnets cannot fund accounts.

Acceptance criteria:

NetworkConfig optional friendbot_url.
network add accepts --friendbot-url.
wallet fund uses network-specific faucet.
Files:

src/utils/config.rs
src/utils/horizon.rs
src/commands/network.rs
src/commands/wallet.rs

[Johnsmichael150]

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Hi Maintainer, I would like to work on this issue

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