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/util/payment_uri.cpp

@@ -115,19 +115,21 @@ 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
     if (result.address[0] == 'z' && result.address.size() >= 78) {
         validFormat = true;
     }
-    // t-address: starts with 'R' (DragonX) or 't' (HUSH) and is ~34 chars  
-    else if ((result.address[0] == 'R' || result.address[0] == 't') && 
+    // t-address: starts with 'R' (DragonX) or 't' (HUSH) and is ~34 chars
+    else if ((result.address[0] == 'R' || result.address[0] == 't') &&
              result.address.size() >= 26 && result.address.size() <= 36) {
         validFormat = true;
     }
-    
+
     if (!validFormat) {
         result.error = "Invalid address format";
         return result;