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_bigpicture.cpp

@@ -0,0 +1,22 @@
+#include "app/run.h"
+#include "platform/types.h"
+
+#if defined(_WIN32)
+#include <windows.h>
+
+int APIENTRY wWinMain(HINSTANCE instance,
+                      HINSTANCE previous_instance,
+                      LPWSTR command_line,
+                      int show_command) {
+    UNREFERENCED_PARAMETER(previous_instance);
+    UNREFERENCED_PARAMETER(command_line);
+
+    const nebula::platform::AppStartup startup{instance, show_command};
+    return nebula::app::RunNebula(startup, {nebula::app::AppMode::BigPicture});
+}
+#else
+int main(int argc, char* argv[]) {
+    const nebula::platform::AppStartup startup{argc, argv};
+    return nebula::app::RunNebula(startup, {nebula::app::AppMode::BigPicture});
+}
+#endif