Fix Windows WebView icon fallback to use executable icon

[ViktorJannicke]

Fixes #708

Summary

Improve the Windows WebView window-icon fallback sequence:

1. Preserve the existing custom icon resource ID 101.
2. Fall back to the executable’s IDI_APPLICATION resource.
3. Fall back to the generic Windows application icon only when neither executable resource exists.

Motivation

WebUI currently loads resource 101 and then immediately uses the generic system icon when that resource is absent.

Windows applications commonly store their executable application icon under IDI_APPLICATION (resource 32512). As a result, an executable can be correctly branded in Explorer while its WebUI window, taskbar button, caption, and Alt+Tab entry display the generic icon.

Using wc.hInstance with IDI_APPLICATION loads the resource from the executable. The existing LoadIcon(NULL, IDI_APPLICATION) call remains the final system fallback.

Compatibility

• Existing applications using resource 101 retain precedence.
• Applications using IDI_APPLICATION now receive their embedded icon.
• Applications without either resource continue to receive the generic system icon.
• No public API or ABI changes are introduced.
• The change only affects the Windows embedded WebView implementation.

Testing

Built successfully on Windows with MSVC as:

• A shared WebUI DLL
• A static WebUI library with WEBUI_WEBVIEW_STATIC

[AlbertShown]

Thank you @ViktorJannicke, much appreciated.