demo secrets and new build scripts

scripts/build.cmd

@@ -0,0 +1,27 @@
+@echo off
+REM Local build-only test script for Windows.
+REM Compiles the project WITHOUT uploading to any device.
+REM Usage: scripts\build.cmd
+
+setlocal
+cd /d "%~dp0\.."
+
+set VENV_DIR=.venv-platformio
+
+if not exist "%VENV_DIR%" (
+    echo Creating local virtualenv in %VENV_DIR% ...
+    python -m venv "%VENV_DIR%"
+    "%VENV_DIR%\Scripts\pip.exe" install --upgrade pip
+    "%VENV_DIR%\Scripts\pip.exe" install platformio
+)
+
+echo Running build (no upload) ...
+"%VENV_DIR%\Scripts\pio.exe" run
+
+if errorlevel 1 (
+    echo Build FAILED.
+    exit /b 1
+)
+
+echo Build finished successfully.
+endlocal