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

ui/js/settings.js

@@ -382,11 +382,11 @@ window.addEventListener('DOMContentLoaded', () => {
     }
     
     if (bigPictureBtn) {
-      bigPictureBtn.addEventListener('click', async () => {
+      bigPictureBtn.addEventListener('click', () => {
         try {
-          if (window.bigPictureAPI && typeof window.bigPictureAPI.launch === 'function') {
+          if (hasNebulaNativeBridge()) {
             showStatus('Launching Big Picture Mode...');
-            await window.bigPictureAPI.launch();
+            window.nebulaNative.postMessage('big-picture');
           } else {
             showStatus('Big Picture Mode not available');
           }