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.cpp

@@ -174,6 +174,11 @@ std::filesystem::path GetCacheDirectory() {
     return cache;
 }
 
+std::filesystem::path GetSessionStatePath() {
+    auto user_data = GetUserDataDirectory();
+    return user_data.empty() ? std::filesystem::path{} : user_data / L"session_state.json";
+}
+
 std::filesystem::path GetUiPagePath(const std::wstring& page_name) {
     const auto exe_dir = GetExecutableDirectory();
     if (exe_dir.empty()) {