Set default window icon from host executable during window creation

[yeelam-gordon]

Summary

WinUI desktop windows show no icon (or the system default) in the taskbar, Alt+Tab, and title bar. This PR fixes the issue by loading the application's icon from the host executable during window creation.

The Problem

As reported in WinUI-Gallery#1512, WinUI apps display the default Windows icon instead of the app's own icon in Alt+Tab / taskbar:

(Screenshot from WinUI-Gallery#1512 -- the WinUI Gallery app shows a default icon in Alt+Tab instead of its actual icon)

Currently the workaround is for every app to manually call AppWindow.SetIcon("path/to/icon.ico") after window creation. But this is:

1. Error-prone -- relative paths resolve against CWD, which differs between VS debugging and installed/packaged apps
2. Unnecessary -- the app's icon is already embedded in the executable as a Win32 resource; WinUI just doesn't use it
3. Not discoverable -- developers expect the icon from Package.appxmanifest or the exe resource to "just work"

Root Cause

DesktopWindowImpl::RegisterDesktopWindowClass() registers WinUIDesktopWin32WindowClass with hIcon = NULL:

WNDCLASSEXW wndClassEx = {};  // zero-initialized -- hIcon and hIconSm are NULL
wndClassEx.cbSize = sizeof(WNDCLASSEX);
wndClassEx.style = CS_HREDRAW | CS_VREDRAW;
wndClassEx.hCursor = LoadCursor(nullptr, IDC_ARROW);
wndClassEx.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
wndClassEx.lpszClassName = s_windowClassName;
// hIcon is NEVER set -- remains NULL

This is because g_hInstance refers to the WinUI framework DLL, not the application's executable. The DLL has no icon resources.

After window creation in OnCreate(), even though the AppWindow object is obtained, no icon is ever loaded or set. Every WinUI window starts with a blank/default icon.

Fix

In OnCreate(), after AppWindow initialization, we now load the app's own icon:

1. Get the host executable's module via GetModuleHandleW(nullptr)
2. Dynamically find the first icon group resource using EnumResourceNamesW with RT_GROUP_ICON
3. Load both large and small icon sizes via LoadImageW with LR_SHARED
4. Set the icon via WM_SETICON

Why this is better than the AppWindow.SetIcon() workaround

	AppWindow.SetIcon() workaround	This fix
Who does the work	Every app developer	WinUI framework (automatic)
Path resolution	Relative paths break in packaged apps	Uses module handle -- no paths involved
Icon source	Requires a separate .ico file	Uses the icon already embedded in the exe
Discoverability	Developer must know to call it	Just works -- icon appears automatically
Override	N/A	Apps can still call AppWindow.SetIcon() to override

Why EnumResourceNamesW instead of a hardcoded resource ID

Different project templates use different icon resource IDs (1, 101, 107, 128, etc.). EnumResourceNamesW(hExeModule, RT_GROUP_ICON, ...) dynamically finds the first icon group -- the same icon the Windows shell displays for the .exe -- regardless of its numeric ID.

Compatibility

Scenario	Before	After
App with icon resource in exe	No icon shown	Correct icon shown
App without icon resource	System default	System default (unchanged -- silently skips)
App calls AppWindow.SetIcon() later	Works	Works (later WM_SETICON overrides)
Fully packaged app	No icon	Icon from exe resource
Sparse-packaged app	No icon	Icon from exe resource
Unpackaged app	No icon	Icon from exe resource
Framework-dependent deployment	No icon	Icon from host exe
Self-contained deployment	No icon	Icon from host exe

Performance

Runs once per window during OnCreate() -- all calls are trivial:

• GetModuleHandleW(nullptr) -- no module load, just returns cached handle
• EnumResourceNamesW -- scans PE resource directory, stops at first hit
• LoadImageW with LR_SHARED -- system-cached
• SendMessageW to own window on same thread -- synchronous, no marshaling

Memory and Thread Safety

• LR_SHARED icons are system-managed -- no DestroyIcon() needed, no leak
• OnCreate() runs synchronously during CreateWindowExW() on the creating thread -- no threading concerns

Fixes #10856
Relates to microsoft/WinUI-Gallery#1512

[yeelam-gordon]

/azp run

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