feat: track shielded send txids via z_viewtransaction

Extract txids from completed z_sendmany operations and store in
send_txids_ so pure shielded sends are discoverable. The network
thread includes them in the enrichment set, calls z_viewtransaction,
caches results in viewtx_cache_, and removes them from send_txids_.

src/app.cpp

@@ -564,13 +564,26 @@ void App::update()
                     };
                 }
             }
-            return [this, done, anySuccess]() {
+            // Extract txids from successful operations so shielded
+            // sends are discoverable by z_viewtransaction.
+            std::vector<std::string> successTxids;
+            for (const auto& op : result) {
+                if (op.value("status", "") == "success"
+                    && op.contains("result") && op["result"].contains("txid")) {
+                    successTxids.push_back(op["result"]["txid"].get<std::string>());
+                }
+            }
+            return [this, done, anySuccess,
+                    successTxids = std::move(successTxids)]() {
                 for (const auto& id : done) {
                     pending_opids_.erase(
                         std::remove(pending_opids_.begin(), pending_opids_.end(), id),
                         pending_opids_.end());
                 }
                 if (anySuccess) {
+                    for (const auto& txid : successTxids) {
+                        send_txids_.insert(txid);
+                    }
                     // Transaction confirmed by daemon — force immediate data refresh
                     transactions_dirty_ = true;
                     addresses_dirty_ = true;