fix(windows): eliminate the white patch behind MSI launch checkbox

[turinglambdaai]

Why

The stock WiX 5 WixUI_InstallDir ExitDialog uses an opaque native Win32 OptionalCheckBox on the finish page. Taskly currently paints a full-size pastel gradient behind it, which creates an ugly rectangular, light-gray/white patch around “Launch taskly” when MSI draws the checkbox and its label. It is an installer UI problem, independent of the Taskly WinUI startup flash handled by #26.

The stock layout positions the optional checkbox at X=135, Y=190, Width=220, Height=40 in a 370×234 dialog-bitmap area (WiX source tag v5.0.2). Taskly's bitmap is 493×312, so the checkbox overlaps approximately x=180–473, y=253–307.

Fix

• Replace the bottom strip of the shipping assets/wizard-dialog.bmp with a quiet #F0F0F0 classic Windows control-face footer, starting at y=248 with a subtle divider at y=247. This blends the launch checkbox into a consistent surface and leaves the existing artwork and brand wordmark intact.
• Synchronize editable SVG and preview PNG with the actual BMP (WiX reads BMP; changing SVG alone is ineffective).
• Add a stdlib-only pixel-level regression check that verifies bitmap dimensions, actual PNG↔BMP pixel identity, and every footer pixel.
• Run the check in CI preflight, Windows packaged acceptance, and Windows release packaging.

Why not replace the checkbox with a fake transparent label?

Windows Installer checkboxes are opaque; splitting a native checkbox into a tiny clickable square plus a transparent Text control makes the label non-clickable and breaks accessibility (screen reader label association). This fix keeps the full real checkbox and its keyboard/screen-reader behavior.

Validation

• Verified WiX 5.0.2 ExitDialog control coordinates and corresponding scaled BMP pixel positions.
• Rendered the revised PNG preview and reviewed the flat footer beneath the artwork.
• CI static bitmap check and Windows build.
• Windows packaged acceptance.
• Manual Windows 10/11 MSI QA: welcome and finish, checked/unchecked Launch, normal and high DPI; verify no rectangular white block or visual discontinuity.

Scope: This PR fixes the native MSI checkbox backdrop, not the application window startup (#26). Standalone Taskly fix without an upstream Rivet dependency bump. No auto-merge.