fix(robustness): guard malformed RPC error JSON + send single-flight (audit #7-8)

- rpc_client::callRaw: a daemon error object is no longer assumed to carry a string
  "message" — a malformed error now yields a clean "RPC error: <dump>" instead of throwing
  a json type-exception from .get<std::string>().
- sendTransaction (full-node): add a single-flight guard so a rapid double-click can't issue
  two z_sendmany before the first returns its opid. The lite path already guarded this; the
  send form guards it in the UI, but the controller entry point now does too.

(#9 from the audit was mostly false positives on verification — all popen sites already
null-check and the xmrig download FILE* path has no throwing calls. The payment-URI
checksum idea was dropped: the send flow already checksum-validates the recipient before
broadcasting, and tightening the parser would reject the placeholder addresses the existing
test relies on; added a comment noting this is format-only by design.)

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

src/app_network.cpp

@@ -2241,6 +2241,15 @@ void App::sendTransaction(const std::string& from, const std::string& to,
         return;
     }
 
+    // Single-flight guard: a rapid double-click (or any second caller) must not issue two
+    // z_sendmany calls before the first returns its opid. The send form already guards this in the
+    // UI, but the controller entry point must not depend on that. (send_submissions_in_flight_ is
+    // main-thread only: ++ here, -- in the worker's main-thread result callback.)
+    if (send_submissions_in_flight_ > 0) {
+        if (callback) callback(false, "A transaction is already being submitted — please wait.");
+        return;
+    }
+
     // Check that we have the spending key for the from address
     if (!from.empty() && from[0] == 'z') {
         bool spendable = false;

src/rpc/rpc_client.cpp

@@ -470,7 +470,14 @@ std::string RPCClient::callRaw(const std::string& method, const json& params)
     // Parse with ordered_json to preserve the daemon's original key order
     nlohmann::ordered_json oj = nlohmann::ordered_json::parse(response_data);
     if (oj.contains("error") && !oj["error"].is_null()) {
-        std::string err_msg = oj["error"]["message"].get<std::string>();
+        // A daemon error object normally has a string "message", but don't assume it — a malformed
+        // error (missing/non-string message) must yield a clean RPC error, not a json type-exception.
+        const auto& err = oj["error"];
+        std::string err_msg;
+        if (err.is_object() && err.contains("message") && err["message"].is_string())
+            err_msg = err["message"].get<std::string>();
+        else
+            err_msg = err.dump();
         throw std::runtime_error("RPC error: " + err_msg);
     }
 

src/util/payment_uri.cpp

@@ -115,7 +115,9 @@ PaymentURI parsePaymentURI(const std::string& uri)
         return result;
     }
     
-    // Basic address format check
+    // Basic address format check. NOTE: this is format-only by design — the send flow
+    // checksum-validates the recipient (isValidBase58Check / shielded check) before broadcasting,
+    // so an invalid-checksum address parsed here can never actually be sent to.
     bool validFormat = false;
 
     // z-address: starts with 'zs' and is 78+ chars