Add NebulaController, tab manager, and CEF clients

Introduce core application structure and browser management: add NebulaController and run entry (src/app/*) to centralize window, tab and CEF lifecycle logic; implement TabManager and NebulaTab (src/browser/*) for tab creation, navigation and state tracking; add URL utilities (NormalizeNavigationInput, JsonEscape) and CEF browser client glue (src/cef/browser_client.cpp/.h) to forward chrome commands and content events. Update app/main.cpp to delegate startup to nebula::app::RunNebula. Add UI assets (chrome.html, chrome.css, chrome.js, lucide, menu-popup updates) and remove obsolete nebot.html. Update CMakeLists to include new sources, add ${CMAKE_SOURCE_DIR}/src to includes and link dwmapi on Windows. Overall this refactors startup and splits responsibilities for cleaner tab and browser lifecycle handling.

src/ui/paths.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <filesystem>
+#include <string>
+
+namespace nebula::ui {
+
+std::filesystem::path GetExecutableDirectory();
+std::filesystem::path GetUiPagePath(const std::wstring& page_name);
+std::string FilePathToUrl(std::filesystem::path path);
+std::string GetChromeUrl();
+std::string GetHomeUrl();
+std::string GetSettingsUrl();
+std::string GetBigPictureUrl();
+std::string GetMenuPopupUrl();
+
+bool IsInternalHomeUrl(const std::string& url);
+bool IsChromiumNewTabUrl(const std::string& url);
+bool IsEmptyOrChromiumNewTabUrl(const std::string& url);
+
+}  // namespace nebula::ui