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/browser/session_state.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "browser/tab.h"
+
+namespace nebula::browser {
+
+struct PersistedTab {
+    std::string url;
+    std::string title = "New Tab";
+};
+
+struct SessionState {
+    std::vector<PersistedTab> tabs;
+    size_t active_tab_index = 0;
+};
+
+SessionState LoadSessionState();
+void SaveSessionState(const std::vector<NebulaTab>& tabs, size_t active_tab_index);
+
+}  // namespace nebula::browser