fix(send): pass the z_sendmany fee as a number, not a string

A prior change passed the user-selected fee to z_sendmany as a fixed-decimal
string (mirroring the recipient amount). But the daemon reads the fee param
with UniValue::get_real(), which rejects a string with "JSON value is not a
number as expected" — breaking every z_sendmany send (surfaced via the
address-to-address transfer feature).

Pass the raw double instead. get_real() parses it directly and accepts any
number notation (including the "5e-05" form of a small fee), so this is
correct for all fee values. The recipient "amount" stays a fixed-decimal
string on purpose — that field is parsed with ParseFixedPoint, which a
scientific-notation double would break.

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

src/app_network.cpp

@@ -2300,16 +2300,18 @@ void App::sendTransaction(const std::string& from, const std::string& to,
 
     send_progress_active_ = true;
     ++send_submissions_in_flight_;
-    // z_sendmany signature is (fromaddress, amounts, minconf, fee). Pass the user-selected
+    // z_sendmany signature is (fromaddress, amounts, minconf, fee). The fee MUST be a JSON number:
-    // fee explicitly — formatted as a fixed-decimal string so the daemon's ParseFixedPoint
+    // the daemon reads it with get_real(), so a string is rejected ("JSON value is not a number as
-    // accepts it (a small double like 0.00005 would serialize to "5e-05" and be rejected).
+    // expected"). get_real() parses the double directly and accepts any number notation (incl. the
-    const std::string fee_str = util::formatAmountFixed(fee);
+    // "5e-05" form of a small fee), so passing the raw double is correct for every fee value.
-    worker_->post([this, from, to, amount, fee_str, memo, recipients, callback]() -> rpc::RPCWorker::MainCb {
+    // (The recipient "amount" above is intentionally a fixed-decimal STRING — that field is parsed
+    // with ParseFixedPoint, which a scientific-notation double would break.)
+    worker_->post([this, from, to, amount, fee, memo, recipients, callback]() -> rpc::RPCWorker::MainCb {
         bool ok = false;
         std::string result_str;
         try {
             rpc::RPCClient::TraceScope trace("Send tab / Submit transaction");
-            auto result = rpc_->call("z_sendmany", {from, recipients, 1, fee_str});
+            auto result = rpc_->call("z_sendmany", {from, recipients, 1, fee});
             result_str = result.get<std::string>();
             ok = true;
         } catch (const std::exception& e) {