chore: upgrade StudioBridge v2.1.2 -> v2.1.3, add persistent-folder creation, update runbook

- Rebuild StudioBridge-2.1.3.jar from latest upstream (v2.1.3) and deploy to portable-dist
- Add ensurePersistentDirectories() so the app creates ~/.studio-bridge and Profiles at startup (GUI + --sendonly)
- Bundle full StudioBridge source under StudioBridge-src/ (with rebuild-jar.bat)
- Update launchers (Launch-StudioBridge.bat, portable-dist/StudioBridge.bat) to v2.1.3
- Add check-update.ps1 and UPDATE.md for the step-by-step update runbook
- Update README (v2.1.3, persistent data + updating sections)
- Ignore StudioBridge-src/target/ build output

StudioBridge-src/rebuild-jar.bat

@@ -0,0 +1,65 @@
+@echo off
+REM ===========================================================================
+REM rebuild-jar.bat - Rebuild StudioBridge.jar from source and deploy it to
+REM the portable-dist folder.
+REM
+REM Requirements:
+REM   - Apache Maven (auto-located from %LOCALAPPDATA%\Programs\apache-maven-*)
+REM   - A JDK/JRE with javac (uses the bundled one in ..\portable-dist\jre)
+REM ===========================================================================
+setlocal
+cd /d "%~dp0"
+
+REM --- Locate Maven ----------------------------------------------------------
+set "MAVEN_CMD="
+for /d %%D in ("%LOCALAPPDATA%\Programs\apache-maven-*") do (
+    if exist "%%D\bin\mvn.cmd" set "MAVEN_CMD=%%D\bin\mvn.cmd"
+)
+if not defined MAVEN_CMD (
+    echo ERROR: Maven not found. Install it or set MAVEN_CMD manually.
+    pause
+    exit /b 1
+)
+
+REM --- Use the bundled JDK/JRE for building ---------------------------------
+set "JAVA_HOME=%~dp0..\portable-dist\jre"
+if not exist "%JAVA_HOME%\bin\javac.exe" (
+    echo ERROR: No compiler found in "%JAVA_HOME%".
+    pause
+    exit /b 1
+)
+
+echo [BUILD] Using Maven : %MAVEN_CMD%
+echo [BUILD] Using JAVA_HOME : %JAVA_HOME%
+echo.
+
+REM --- Build ----------------------------------------------------------------
+call "%MAVEN_CMD%" -f pom.xml clean package
+if errorlevel 1 (
+    echo.
+    echo ERROR: Maven build failed.
+    pause
+    exit /b 1
+)
+
+REM --- Deploy the fat jar to portable-dist ----------------------------------
+set "JAR_VERSION=2.1.3"
+set "BUILT=target\StudioBridge-%JAR_VERSION%-jar-with-dependencies.jar"
+set "DEST=..\portable-dist\StudioBridge-%JAR_VERSION%.jar"
+
+if not exist "%BUILT%" (
+    echo ERROR: Build output not found: %BUILT%
+    pause
+    exit /b 1
+)
+
+copy /Y "%BUILT%" "%DEST%" >nul
+if errorlevel 1 (
+    echo ERROR: Could not copy jar to %DEST%
+    pause
+    exit /b 1
+)
+
+echo.
+echo [OK] Rebuilt and deployed: %DEST%
+endlocal