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

src/cef/browser_client.cpp

@@ -111,6 +111,7 @@ bool NebulaBrowserClient::OnProcessMessageReceived(CefRefPtr<CefBrowser> browser
         const bool allowed_settings_command =
             IsSettingsFrame(frame) && (command == "navigate" ||
                                        command == "new-tab" ||
+                                       command == "big-picture" ||
                                        command == "check-default-browser" ||
                                        command == "set-default-browser" ||
                                        command == "clear-site-history" ||