fix: Tier-2 involved mediums — send confirm re-validation + console JSON errors

- Send: re-validate against LIVE state at the "Confirm & Send" click (connected,
  not syncing, amount > 0, total <= available). The confirm dialog persists
  across frames, so a balance drop / sync restart / fee bump after Review could
  otherwise broadcast a now-invalid transaction; it now shows a clear message
  instead of sending.
- Console: a malformed JSON argument ({...}/[...] that fails to parse) was
  silently sent to the daemon as a plain string. Now the command fails with a
  clear "Argument N is not valid JSON" line instead of being silently mangled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/console_input_model.h

@@ -17,6 +17,7 @@ struct ConsoleRpcCall {
     bool valid = false;
     std::string method;
     nlohmann::json params = nlohmann::json::array();
+    std::string error;   // set (with valid=false) when an argument fails to parse
 };
 
 enum class ConsoleResultLineRole {