fix(onboard): lower flash baud rate to 115200 to prevent hang on embedded-flash boards

460800 causes the stub to hang mid-write on ESP32-S3 QFN56 boards with XMC
embedded flash. 115200 skips the post-stub baud rate negotiation entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/js/onboard.js

@@ -504,7 +504,7 @@
                 transport = new Transport(state.port, false);
                 var loader = new ESPLoader({
                     transport: transport,
-                    baudrate: 460800,
+                    baudrate: 115200,
                     terminal: {
                         clean: function () {},
                         writeLine: function (s) { appendLog('log', [s]); },