[Enhancement] Integrate bdk-floresta as a chain source

[4xvgal]

Describe the enhancement
Add FlorestaChainSource that uses the bdk-floresta FFI bridge for blockchain data, transaction broadcast, and fee estimation.

Depends on:

• [Enhancement] Add real fee estimation to Floresta core and expose it via libfloresta APIs getfloresta/Floresta#1141
• [Enhancement] Add Node::estimate_fee() to bdk-floresta luisschwab/bdk-floresta#140
• This issue also depends on the FFI bridge issue in coconut_wallet.

Responsibilities:

class FlorestaChainSource implements ChainSource {
  Future<List<Utxo>> getUtxos(ScriptPubKey script);
  Future<List<Tx>> getHistory(ScriptPubKey script);
  Future<String> broadcast(String txHex);
  Future<int> getBlockHeight();
  Future<FeeEstimate> getFeeEstimate();
}
Use case
Once this lands, coconut_wallet can run a lightweight full node locally without a remote Electrum server.
Additional context
- Descriptor registration, scanning, and rescan should run in the isolate layer.
- Electrum integration should remain as a fallback.