Add Windows app icon resource & load it

Add Windows resource files and load the app icon in the window class. Introduce resources/nebula.rc and resources/resource.h (defines IDI_APPICON referencing assets/images/branding/Nebula-Favicon.ico). Include resource.h in nebula_window_win.cpp and set window_class.hIcon/hIconSm via LoadImageW. Update CMakeLists.txt to compile the .rc and add the resources include directory for Windows.

src/platform/win/nebula_window_win.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <string_view>
 
+#include "resource.h"
 #include "ui/paths.h"
 
 namespace nebula::window {
@@ -426,6 +427,20 @@ void nebula::window::NebulaWindowImpl::RegisterClass(HINSTANCE instance_handle)
     window_class.lpfnWndProc = StaticWndProc;
     window_class.hInstance = instance_handle;
     window_class.hCursor = LoadCursor(nullptr, IDC_ARROW);
+    window_class.hIcon = static_cast<HICON>(LoadImageW(
+        instance_handle,
+        MAKEINTRESOURCEW(IDI_APPICON),
+        IMAGE_ICON,
+        GetSystemMetrics(SM_CXICON),
+        GetSystemMetrics(SM_CYICON),
+        LR_DEFAULTCOLOR));
+    window_class.hIconSm = static_cast<HICON>(LoadImageW(
+        instance_handle,
+        MAKEINTRESOURCEW(IDI_APPICON),
+        IMAGE_ICON,
+        GetSystemMetrics(SM_CXSMICON),
+        GetSystemMetrics(SM_CYSMICON),
+        LR_DEFAULTCOLOR));
     window_class.lpszClassName = WindowClassName();
 
     RegisterClassExW(&window_class);