Enable GPU/WebGL and add persistent cache dirs

Enable hardware-accelerated rendering and persist GPU/cache data. Added a BrowserSettings() helper that enables WebGL and use it when creating Chrome/Content/MenuPopup browsers (src/app/nebula_controller.cpp). Configure CefSettings to use a persistent user data and cache directory (src/app/run.cpp) by calling nebula::ui::GetUserDataDirectory() and GetCacheDirectory(). Add command-line switches to initialize the GPU process and avoid sandbox/blocklist fallbacks (disable GPU sandbox, in-process-gpu, ignore-gpu-blocklist, enable-accelerated-video-decode, use ANGLE D3D11) to prevent GPU crashes and Chromium falling back to software rendering (src/cef/nebula_app.cpp). Implement GetUserDataDirectory() and GetCacheDirectory() (preferring %LOCALAPPDATA% with an executable-directory fallback) and expose them in the header (src/ui/paths.cpp, src/ui/paths.h). These changes ensure GPU shader caching, WebGL support, and smoother video/graphics behavior.

src/ui/paths.h

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