Set the Windows title-bar icon from the embedded exe resource

[turinglambdaai]

Problem

Every rivet Windows app runs with WinUI's generic placeholder icon in the title bar, while the taskbar, Alt-Tab and shortcuts show the real brand icon. Found while triaging podlens: the exe correctly carries the project icon (embedded by rivet-cli/build.rkt at resource ID 1 since #283), but nothing ever loaded it into the AppWindow — WinUI only uses the exe icon for the taskbar; the title bar needs an explicit AppWindow.SetIcon.

Screenshot before (podlens, title bar at 16px): the slot left of the title shows WinUI's teal default glyph.

Fix

The host template's MainWindow constructor now loads the embedded icon and hands it to the window:

if (HICON const icon =
        ::LoadIconW(::GetModuleHandleW(nullptr), MAKEINTRESOURCEW(1))) {
  AppWindow().SetIcon(winrt::Microsoft::UI::IconId{
      static_cast<std::uint64_t>(reinterpret_cast<UINT_PTR>(icon))});
}

• Uses the IconId overload, so no icon file needs to ship beside the exe (no resources staging needed).
• Null guard: projects without a windows-icon declaration keep the placeholder.
• Verified end to end in podlens (built with the linked checkout at a701e35): the title bar now shows the terracotta brand tile; podlens landed the same lines in its own MainWindow (de7149e) as the first existing product.

Existing products (taskly, payback, movebit, …) carry their own generated MainWindow.xaml.cpp and need the same lines; the template fix covers all newly scaffolded hosts.

Testing

• raco test rivet/tests/ (no Racket-side change expected)
• Compiled and run in podlens against the linked checkout; title-bar icon verified visually at native resolution.