fix(tx): track async operations to completion (send/shield/auto-shield)

z_sendmany returns an opid immediately; the tx is built/signed/broadcast
asynchronously afterward. The send path showed "Transaction sent successfully!"
and cleared the form on opid receipt, so a later async failure contradicted it.
Shield/merge stored the opid only in a dialog-local static (never polled), and
auto-shield ran a blocking z_shieldcoinbase on the UI thread and discarded its
opid — async failures of all three were silently lost.

- Add App::trackOperation(opid) so shield/merge/auto-shield register with the
  shared opid poller (failures surface, balances refresh on completion).
- Defer the full-node send's success/failure to the poller via per-opid callbacks
  (parseOperationStatusPoll now exposes failureByOpid); the "Sending..." spinner
  covers the finalizing window, and the form is kept until terminal status.
- Dispatch auto-shield through the worker thread and use the configured fee.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -250,11 +250,18 @@ public:
     // Wallet backup
     void backupWallet(const std::string& destination, std::function<void(bool, const std::string&)> callback);
 
-    // Transaction operations  
-    void sendTransaction(const std::string& from, const std::string& to, 
+    // Transaction operations
+    void sendTransaction(const std::string& from, const std::string& to,
                         double amount, double fee, const std::string& memo,
                         std::function<void(bool success, const std::string& result)> callback);
 
+    // Register a daemon async operation id (z_shieldcoinbase / z_mergetoaddress /
+    // auto-shield) with the shared opid poller so its eventual success/failure is
+    // surfaced and balances/transactions refresh on completion. z_sendmany uses the
+    // richer pending-send path internally; this is for operations with no optimistic
+    // transaction row of their own.
+    void trackOperation(const std::string& opid);
+
     // Force refresh
     void refreshNow();
     void refreshMiningInfo();
@@ -399,6 +406,10 @@ private:
                                              const std::string& txid);
     void removePendingSendTransactions(const std::vector<std::string>& opids,
                                        bool restoreBalances);
+    // Deliver a deferred z_sendmany result to its waiting UI callback once the opid
+    // reaches a terminal status. Returns true if a callback was registered (and fired).
+    bool invokeSendResultCallback(const std::string& opid, bool ok,
+                                  const std::string& result);
     void applyPendingSendBalanceDeltas(bool includeAggregateBalances);
     std::string transactionHistoryCacheWalletIdentity() const;
     bool ensureTransactionHistoryCacheUnlockedFor(const std::string& walletIdentity);
@@ -576,6 +587,10 @@ private:
         std::int64_t timestamp = 0;
     };
     std::unordered_map<std::string, PendingSendInfo> pending_send_info_;
+    // z_sendmany UI callbacks held until the opid reaches a terminal status, so the
+    // user isn't told "sent successfully" before the tx is actually built/broadcast.
+    std::unordered_map<std::string, std::function<void(bool, const std::string&)>>
+        pending_send_callbacks_;
     // Txids from completed z_sendmany operations.
     // Ensures shielded sends are discoverable by z_viewtransaction
     // even when they don't appear in listtransactions or