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/cef/nebula_app.cpp

@@ -58,6 +58,26 @@ void NebulaApp::OnBeforeCommandLineProcessing(const CefString& process_type,
 
     // The bundled UI is loaded from file:// and uses ES modules.
     command_line->AppendSwitch("allow-file-access-from-files");
+
+    // CefSettings.no_sandbox disables the browser-level sandbox, but Chromium
+    // still attempts to bring up a separate GPU sandbox inside the GPU process.
+    // Without the host-side sandbox plumbing this fails with STATUS_BREAKPOINT
+    // (-2147483645) immediately on startup, which is exactly what the GPU
+    // diagnostics page was showing - the GPU process crashed three times and
+    // Chromium then fell back to software rendering. Disabling the GPU sandbox
+    // matches the rest of our no_sandbox configuration and lets the GPU
+    // process initialize.
+    command_line->AppendSwitch("no-sandbox");
+    command_line->AppendSwitch("disable-gpu-sandbox");
+    command_line->AppendSwitch("in-process-gpu");
+
+    // Avoid Chromium's conservative GPU blocklist, but let Chromium choose the
+    // safest graphics backend for this machine. Forcing raster/zero-copy paths
+    // can prevent WebGL shared contexts from initializing on some drivers.
+    command_line->AppendSwitch("ignore-gpu-blocklist");
+    command_line->AppendSwitch("enable-accelerated-video-decode");
+    command_line->AppendSwitchWithValue("use-gl", "angle");
+    command_line->AppendSwitchWithValue("use-angle", "d3d11");
 }
 
 void NebulaApp::OnContextCreated(CefRefPtr<CefBrowser> browser,