Add Big Picture mode and multi-target build

Introduce a Big Picture mode and support building two app targets. CMakeLists was refactored to add a helper (add_nebula_app_target) and now registers NebulaBrowser and NebulaBigPicture executables, moving UI/assets post-build copying into the helper. A new app/main_bigpicture.cpp entry was added and RunNebula now accepts LaunchOptions (AppMode) with a new LaunchOptions struct. NebulaController was extended heavily to manage a BigPicture browser role: creation, enter/exit mode, layout logic, cursor injection/removal, remote input handlers (mouse move/click/wheel/text), and state syncing. Cef/browser_client updated for the new BigPicture role and message filtering. Platform API gained MoveCursorToBrowserPoint with platform stubs/Win implementation. Big-picture UI files (CSS/JS/HTML) were also updated to support the new mode.

app/main.cpp

@@ -12,11 +12,11 @@ int APIENTRY wWinMain(HINSTANCE instance,
     UNREFERENCED_PARAMETER(command_line);
 
     const nebula::platform::AppStartup startup{instance, show_command};
-    return nebula::app::RunNebula(startup);
+    return nebula::app::RunNebula(startup, {nebula::app::AppMode::Desktop});
 }
 #else
 int main(int argc, char* argv[]) {
     const nebula::platform::AppStartup startup{argc, argv};
-    return nebula::app::RunNebula(startup);
+    return nebula::app::RunNebula(startup, {nebula::app::AppMode::Desktop});
 }
 #endif