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/ui/paths.h

@@ -8,6 +8,7 @@ namespace nebula::ui {
 std::filesystem::path GetExecutableDirectory();
 std::filesystem::path GetUserDataDirectory();
 std::filesystem::path GetCacheDirectory();
+std::filesystem::path GetSessionStatePath();
 std::filesystem::path GetUiPagePath(const std::wstring& page_name);
 std::string FilePathToUrl(std::filesystem::path path);
 std::string GetChromeUrl();