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.

src/app/run.cpp

@@ -9,7 +9,7 @@
 
 namespace nebula::app {
 
-int RunNebula(const nebula::platform::AppStartup& startup) {
+int RunNebula(const nebula::platform::AppStartup& startup, LaunchOptions options) {
     nebula::platform::PrepareApp();
 
     const CefMainArgs main_args = nebula::platform::MakeMainArgs(startup);
@@ -41,7 +41,7 @@ int RunNebula(const nebula::platform::AppStartup& startup) {
         initial_url = nebula::ui::GetHomeUrl();
     }
 
-    NebulaController controller(startup, std::move(initial_url));
+    NebulaController controller(startup, std::move(initial_url), options);
     const bool created = controller.Create();
     if (created) {
         CefRunMessageLoop();