feat(wallet): enforce confirmation thresholds before spending UTXOs

Coinbase outputs require COINBASE_MATURITY + 1 = 121 confirmations before
they can be spent. The consensus rule (palladiumcore tx_verify.cpp) is
nSpendHeight - nHeight >= 120; the +1 adds one block of safety margin,
matching the Qt wallet (wallet.cpp: COINBASE_MATURITY+1). Regular UTXOs
require MinConfirmations (6 on mainnet, 1 on testnet/regtest — wallet
policy, no consensus rule).

Changes:
- ChainProfile: add CoinbaseMaturity and MinConfirmations fields
- ChainProfiles: mainnet CoinbaseMaturity=120 / MinConfirmations=6;
  testnet/regtest inherit 120, override MinConfirmations=1
- PalladiumNetworks: align NBitcoin Consensus.CoinbaseMaturity to 120
- CachedUtxo: add IsCoinbase flag (ElectrumX listunspent does not expose
  this; derived from Transaction.IsCoinBase on the raw tx)
- WalletSynchronizer: set IsCoinbase during local UTXO reconstruction
- TransactionFactory.Build: add tipHeight param; filter by threshold;
  error reports reason (immature coinbase with X/121 counter, under-
  confirmed regulars with best-seen count, mempool amounts)
- All Build() call sites updated (App, Donate, CLI)
- Two new tests: coinbase maturity boundary (119→220 tip) and mainnet
  min-conf boundary (5→6 confirmations)

src/App/ViewModels/MainWindowViewModel.Donate.cs

@@ -49,7 +49,7 @@ public partial class MainWindowViewModel
 
             _pendingDonate = new TransactionFactory(_account).Build(
                 _doc.Cache.Utxos, _lastTransactions, destination, amount, feeRate,
-                _doc.Cache.NextChangeIndex, sendAll: false);
+                _doc.Cache.NextChangeIndex, _doc.Cache.TipHeight, sendAll: false);
 
             DonatePreview = $"txid {_pendingDonate.Txid[..16]}… · " +
                 $"fee {Fmt(_pendingDonate.Fee.Satoshi)} " +