fix: update platform dependency and fix breaking API changes

Update dashpay/platform rev to fc732a9 and rust-dashcore rev to 9959201.
Fix three breaking changes:
- FeeLevel enum removed → use FeeRate::normal() static constructor
- set_fee_level() removed → use set_fee_rate()
- NetworkExt trait removed → methods now inherent on Network

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lklimek