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/ui/paths.h

@@ -2,9 +2,15 @@
 
 #include <filesystem>
 #include <string>
+#include <string_view>
 
 namespace nebula::ui {
 
+// Process-wide profile used to isolate desktop vs Big Picture CEF data.
+// Call before GetUserDataDirectory() / CEF initialize. Empty/"desktop" = default.
+void SetAppProfile(std::string_view profile);
+const std::string& GetAppProfile();
+
 std::filesystem::path GetExecutableDirectory();
 std::filesystem::path GetUserDataDirectory();
 std::filesystem::path GetCacheDirectory();