Introduce default-browser integration and external open handling across platforms. Added platform/default_browser.h with a Windows implementation (registry registration + settings UI invocation) and mac/linux stubs. Exposed new commands from the renderer (check-default-browser / set-default-browser) and implemented request/response plumbing in NebulaController (SendDefaultBrowserResult) and BrowserClient. Added UI controls and JS helpers in settings and setup pages to check and prompt the user to make Nebula the default browser.
Also added single-instance launch target handling: command-line URL normalization, passing/consolidating the launch target, forwarding it to an existing window on Windows via WM_COPYDATA, and exposing OnExternalOpenRequested on the window/controller. Implemented delayed navigation (CefTask) to safely load pending initial URLs after CEF initialization. Updated CMakeLists and platform startup signatures to include and accept the new files/parameters.
Introduce a cross-platform platform layer and port scaffolding for macOS and Linux. CMakeLists.txt refactored to select platform sources, set executable type per OS, and use CEF helper macros for runtime deployment. Add platform/types.h, startup/paths/browser_host APIs and implementations for Windows, macOS, and Linux (many are stubs for mac/linux). Refactor app entry and lifetime to use nebula::platform::AppStartup (app/main, run.{h,cpp}), move window/browser host logic into platform/browser_host.*, and update NebulaController to use platform APIs (native handles, sizing, visibility, cache-busting token, etc.). Add README and detailed docs/cross-platform.md describing build layout and porting status.
Andrew Zambazos