Persist session state and single-instance

Add session persistence and single-instance handling. Introduces browser/session_state.{h,cpp} to load/save a simple JSON session_state.json (limits restored tabs to 50, basic JSON parsing, atomic write via a .tmp rename). TabManager gains RestoreTabs and ActiveTabIndex to restore and track tabs. NebulaController now calls PersistSession on tab/title/activate/close events, flushes cookies on shutdown, and sets CEF runtime style to Alloy for embedded child browsers and devtools. run.cpp adds a named mutex to prevent multiple instances, enables persistent session cookies, and tweaks initial URL handling. Added GetSessionStatePath() to ui/paths and updated CMakeLists.txt to include the new source file.

src/app/nebula_controller.h

@@ -54,6 +54,7 @@ private:
     void SetContentFullscreen(bool fullscreen);
     void ResizeBrowsers();
     void SendChromeState(const nebula::browser::NebulaTab& tab);
+    void PersistSession() const;
     void MaybeFinishShutdown();
 
     HINSTANCE instance_ = nullptr;