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/Core/Storage/WalletStore.cs

@@ -37,4 +37,12 @@ public static class WalletStore
         File.WriteAllText(tmp, content);
         File.Move(tmp, path, overwrite: true);
     }
+
+    /// <summary>
+    /// Same as <see cref="Save"/> but with the JSON serialization and disk write off the
+    /// calling thread — for callers on a UI thread saving a large cache (thousands of cached
+    /// transactions/headers), where the synchronous version would block the UI.
+    /// </summary>
+    public static Task SaveAsync(WalletDocument doc, string path, string? password = null) =>
+        Task.Run(() => Save(doc, path, password));
 }