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/platform/mac/browser_host_mac.cpp

@@ -32,6 +32,12 @@ void RaiseBrowserWindow(NativeWindow browser_window) {
     UNREFERENCED_PARAMETER(browser_window);
 }
 
+void MoveCursorToBrowserPoint(NativeWindow browser_window, int x, int y) {
+    UNREFERENCED_PARAMETER(browser_window);
+    UNREFERENCED_PARAMETER(x);
+    UNREFERENCED_PARAMETER(y);
+}
+
 int ScaleForParentWindow(NativeWindow parent, int value) {
     UNREFERENCED_PARAMETER(parent);
     return value;