feat(spv): verify Merkle proofs progressively, gate spendability on it

Balance/history now render as soon as tx downloads finish instead of
blocking on every historical Merkle proof, critical for mobile where
proof-checking can take much longer than the download itself. Proofs
continue to be checked in the background and each tx's Verified flag
catches up progressively; header ranges are now fetched in batches
(blockchain.block.headers) instead of one call per header to keep this
fast over high-latency links.

Coin selection (UtxoSpendability.IsSpendable) refuses to spend a UTXO
until its Merkle proof is actually checked, regardless of confirmation
count, so a server that fabricates a confirmed balance can get it
displayed early but never spent before the forgery is caught. The disk
cache only ever persists the fully-verified end state of a sync.

UI surfaces the new PendingVerificationSats/SpendableSats split with a
"verifying..." badge, and the sync save now runs off the UI thread to
avoid freezing on slower hardware.

src/App/ViewModels/MainWindowViewModel.cs

@@ -17,7 +17,7 @@ using PalladiumWallet.Core.Wallet;
 namespace PalladiumWallet.App.ViewModels;
 
 /// <summary>Transaction history row for the view.</summary>
-public sealed record HistoryRow(string Conferma, string Importo, string Txid);
+public sealed record HistoryRow(string Conferma, string Importo, string Txid, bool Verified = true);
 
 /// <summary>Address view row with pre-computed keys and derivation path.</summary>
 public sealed record AddressRow(