Support running Big Picture as separate process

Enable Big Picture mode to run concurrently with Desktop mode by:

- Implementing cross-platform process launching (LaunchSiblingExecutable) for Windows, macOS, and Linux
- Adding app profile system to isolate CEF user data between modes
- Making window class/title and mutex names profile-aware
- Changing mode switching to launch sibling executable instead of in-process switching
- Updating settings.js to use native bridge for Big Picture launch
- Adding build.bat helper for Windows builds

build.bat

@@ -0,0 +1,53 @@
+@echo off
+setlocal
+
+rem Build Nebula Browser (Windows).
+rem Usage:
+rem   build.bat              - configure + Release build
+rem   build.bat Debug        - configure + Debug build
+rem   build.bat Release      - configure + Release build
+rem   build.bat configure    - configure only
+rem   build.bat clean        - remove the build folder
+
+cd /d "%~dp0"
+
+set "CONFIG=Release"
+if /I "%~1"=="Debug" set "CONFIG=Debug"
+if /I "%~1"=="Release" set "CONFIG=Release"
+
+if /I "%~1"=="clean" (
+    echo Removing build\ ...
+    if exist build rmdir /s /q build
+    echo Done.
+    exit /b 0
+)
+
+if not exist "thirdparty\cef\cmake\FindCEF.cmake" (
+    echo ERROR: CEF not found at thirdparty\cef\
+    echo Download a Windows 64-bit CEF standard binary distribution and extract it
+    echo so that thirdparty\cef\cmake\FindCEF.cmake exists.
+    exit /b 1
+)
+
+echo Configuring CMake...
+cmake -B build
+if errorlevel 1 (
+    echo Configure failed.
+    exit /b 1
+)
+
+if /I "%~1"=="configure" (
+    echo Configure complete.
+    exit /b 0
+)
+
+echo Building %CONFIG%...
+cmake --build build --config %CONFIG%
+if errorlevel 1 (
+    echo Build failed.
+    exit /b 1
+)
+
+echo.
+echo Build succeeded: build\%CONFIG%\NebulaBrowser.exe
+exit /b 0