Windows tray icons incorrectly load multi-image ICO files #5961

Description

On Windows, SystemTray.SetIcon and application/window icon loading handle PNG bytes correctly but pass a complete multi-image ICO container directly to CreateIconFromResourceEx. That API expects the bytes of one RT_ICON image resource, not the ICO file header, directory, and all image entries. As a result, .ico files can fail to load or render/select incorrectly in the system tray.

Reproduction

1. Embed a conventional multi-size ICO containing 16, 32, 48, and/or 256 pixel images.
2. Pass its bytes to app.SystemTray.New().SetIcon(icon) on Windows.
3. Run the application and inspect the tray icon.

//go:embed icon.ico
var icon []byte

tray := app.SystemTray.New()
tray.SetIcon(icon)

Root cause

v3/pkg/w32/icon.go recognizes the ICO signature, but CreateSmallHIconFromImage and CreateLargeHIconFromImage pass the entire file buffer to CreateIconFromResourceEx. ICO files need to be parsed and the best matching ICONDIRENTRY payload extracted first.

Scope

• Parse the ICO header and directory safely.
• Select the best image for the requested Windows system metric, preferring the closest size that does not exceed it and then color depth.
• Pass only the selected PNG/DIB image payload to CreateIconFromResourceEx.
• Preserve direct PNG support.
• Cover multi-image selection, 256-pixel entries, malformed bounds, and actual HICON creation with tests.

Acceptance criteria

• Multi-image ICO bytes produce a non-null Windows HICON.
• The closest suitable image entry is selected deterministically.
• Malformed ICO offsets and lengths return an error without panicking.
• Existing PNG icon loading continues to work.